Added contributing guide (#6)

Author: hiranya911

CONTRIBUTING.md

@@ -0,0 +1,137 @@
+# Contributing | Firebase Admin .NET SDK
+
+Thank you for contributing to the Firebase community!
+
+ - [Have a usage question?](#question)
+ - [Think you found a bug?](#issue)
+ - [Have a feature request?](#feature)
+ - [Want to submit a pull request?](#submit)
+ - [Need to get set up locally?](#local-setup)
+
+
+## <a name="question"></a>Have a usage question?
+
+We get lots of those and we love helping you, but GitHub is not the best place for them. Issues
+which just ask about usage will be closed. Here are some resources to get help:
+
+- Go through the [guides](https://firebase.google.com/docs/admin/setup/)
+- Read the full [API reference](https://firebase.google.com/docs/reference/admin/)
+
+If the official documentation doesn't help, try asking a question on the
+[Firebase Google Group](https://groups.google.com/forum/#!forum/firebase-talk/) or one of our
+other [official support channels](https://firebase.google.com/support/).
+
+**Please avoid double posting across multiple channels!**
+
+
+## <a name="issue"></a>Think you found a bug?
+
+Yeah, we're definitely not perfect!
+
+Search through [old issues](https://github.com/firebase/firebase-admin-dotnet/issues) before
+submitting a new issue as your question may have already been answered.
+
+If your issue appears to be a bug, and hasn't been reported,
+[open a new issue](https://github.com/firebase/firebase-admin-dotnet/issues/new). Please use the
+provided bug report template and include a minimal repro.
+
+If you are up to the challenge, [submit a pull request](#submit) with a fix!
+
+
+## <a name="feature"></a>Have a feature request?
+
+Great, we love hearing how we can improve our products! Share you idea through our
+[feature request support channel](https://firebase.google.com/support/contact/bugs-features/).
+
+
+## <a name="submit"></a>Want to submit a pull request?
+
+Sweet, we'd love to accept your contribution!
+[Open a new pull request](https://github.com/firebase/firebase-admin-dotnet/pull/new/master) and fill
+out the provided template.
+
+**If you want to implement a new feature, please open an issue with a proposal first so that we can
+figure out if the feature makes sense and how it will work.**
+
+Make sure your changes pass our linter and the tests all pass on your local machine. We've hooked
+up this repo with continuous integration to double check those things for you.
+
+Most non-trivial changes should include some extra test coverage. If you aren't sure how to add
+tests, feel free to submit regardless and ask us for some advice.
+
+Finally, you will need to sign our
+[Contributor License Agreement](https://cla.developers.google.com/about/google-individual),
+and go through our code review process before we can accept your pull request.
+
+### Contributor License Agreement
+
+Contributions to this project must be accompanied by a Contributor License
+Agreement. You (or your employer) retain the copyright to your contribution.
+This simply gives us permission to use and redistribute your contributions as
+part of the project. Head over to <https://cla.developers.google.com/> to see
+your current agreements on file or to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one
+(even if it was for a different project), you probably don't need to do it
+again.
+
+### Code reviews
+
+All submissions, including submissions by project members, require review. We
+use GitHub pull requests for this purpose. Consult
+[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
+information on using pull requests.
+
+
+## <a name="local-setup"></a>Need to get set up locally?
+
+### Initial Setup
+
+Run the following commands from the command line to get your local environment set up:
+
+```bash
+$ git clone https://github.com/firebase/firebase-admin-dotnet.git
+$ cd firebase-admin-dotnet/FirebaseAdmin # Change into the FirebaseAdmin solution directory
+$ dotnet restore                         # Install dependencies 
+```
+
+### Running Tests
+
+There are two test suites, housed under two separate subdirectories:
+
+* `FirebaseAdmin/FirebaseAdmin.Tests`: Unit tests
+* `FirebaseAdmin/FirebaseAdmin.IntegrationTests`: Integration tests
+
+The unit test suite is intended to be run during development, and the integration test suite is
+intended to be run before packaging up release candidates.
+
+To run the unit test suite:
+
+```bash
+$ dotnet test FirebaseAdmin.Tests
+```
+
+The integration test suite requires a service account JSON key file, and an API key for a Firebase
+project. Create a new project in the [Firebase console](https://console.firebase.google.com) if
+you do not already have one. Use a separate, dedicated project for integration tests since the
+test suite makes a large number of writes to the Firebase realtime database. Download the service
+account key file from the "Settings > Service Accounts" page of the project, and copy it to
+`FirebaseAdmin/FirebaseAdmin.IntegrationTests/resources/integration_cert.json`. Also obtain the
+API key for the same project from "Settings > General", and save it to
+`FirebaseAdmin/FirebaseAdmin.IntegrationTests/resources/integration_apikey.txt`. Finally, to run
+the integration test suite:
+
+```bash
+$ dotnet test FirebaseAdmin.IntegrationTests
+```
+
+### Repo Organization
+
+Here are some highlights of the directory structure and notable source files
+
+* `FirebaseAdmin/` - Solution directory containing all source code and tests.
+  * `FirebaseAdmin.sln/` - Visual Studio solution file for the project.
+  * `FirebaseAdmin/` - Source directory.
+  * `FirebaseAdmin.Tests/` - Unit tests directory.
+  * `FirebaseAdmin.IntegrationTests/` - Integrarion tests directory. 
+