Added contributing guidelines file plus reference in readme (#90)

## Summary
Added contributing.md as well as reference from readme to it.

## How was it tested?
Not tested - only documentation change.

Author: mohsenari

CONTRIBUTING.md

@@ -0,0 +1,90 @@
+# Contributing
+
+When contributing to this repository, please describe the change you wish to make via a related issue, or a pull request.
+
+Please note we have a [code of conduct](CODE_OF_CONDUCT.md), please follow it in all your interactions with the project.
+
+## Setting Up Development Environment
+Before making any changes to the source code (documentation excluded) make sure you have installed all the required tools. 
+### Prerequisites
+* Install [Nix Package Manager](https://nixos.org/download.html).
+* Install [Golang](https://go.dev/doc/install) (current version: 1.19)
+* Clone this repository: 
+    * ```bash
+        git clone github.com/jetpack/devbox go.jetpack.io
+        ```
+* Setup you `GOPATH` env variable to the parent directory of `go.jetpack.io/` 
+    * Example: If the cloned repository is at `/Users/johndoe/projects/go.jetpack.io/`:
+        ```bash
+        export GOPATH=/Users/johndoe/projects/
+        ```
+
+## Building and Testing
+Devbox is setup like a typical Go project. After installing the required tools and setting up your environment. You can make changes in the source code, build, and test your changes by following these steps:
+
+1. Install dependencies:
+    ```bash
+    go install
+    ```
+2. Build Devbox:
+    ```bash
+    go build -o ./devbox cmd/devbox/main.go
+    ```
+    This will build an executable file.
+3. Run and test Devbox:
+    ```bash
+    ./devbox <your_test_command>
+    ```
+
+
+## Pull Request Process
+
+1. Ensure any new feature or functionality also includes tests to verify its correctness.
+
+2. Ensure any new dependency is also included in [go.mod](go.mod) file
+
+2. Ensure any binary file as a result of build (e.g., `./devbox`) are removed and/or excluded from tracking in git.
+
+3. Update the [README.md](README.md) and/or docs with details of changes to the interface, this includes new environment 
+   variables, new commands, new flags, and useful file locations.
+
+4. You may merge the Pull Request in once you have the sign-off of developers/maintainers, or if you 
+   do not have permission to do that, you may request the maintainers to merge it for you.
+
+## Developer Certificate of Origin
+By contributing to this project you agree to the [Developer Certificate of Origin](https://developercertificate.org/) (DCO) which was created by the Linux Foundation and is a simple statement that you, as a contributor, have the legal right to make the contribution. See the DCO description for details below:
+> Developer Certificate of Origin
+> 
+> Version 1.1
+>
+> Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+>
+> Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+>
+>
+> Developer's Certificate of Origin 1.1
+>
+> By making a contribution to this project, I certify that:
+> 
+> (a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+> 
+> (b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+> 
+> (c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+>
+> (d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.

README.md

@@ -162,6 +162,9 @@ See the [CLI Reference](https://www.jetpack.io/devbox/docs/cli_reference/) for t
 + File bug reports and feature requests using [Github Issues](https://github.com/jetpack-io/devbox/issues)
 + Follow us on [Jetpack's Twitter](https://twitter.com/jetpack_io) for product updates
 
+## Contributing
+Devbox is an opensource project so contributions are always welcome. Please read [our contributing guide](CONTRIBUTING.md) before submitting pull requests.
+
 ## Related Work
 
 Thanks to [Nix](https://nixos.org/) for providing isolated shells.