docs: add contributing guide to library (#164)

* docs: add contributing guide to library

* docs: address comments

Author: ebk45

CONTRIBUTING.md

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+# Contributing to the github-java-client
+
+Thanks for your interest in the github-java-client. Our goal is to bring a reliable Java-based 
+alternative to the GitHub API.
+
+## Getting Started
+
+The Github Java Clients's [open issues are here](https://github.com/github-java-client/github-java-client/issues). 
+In time, we'll tag issues that would make a good first pull request for new contributors. An easy 
+way to get started helping the project is to *file an issue*. Issues can include bugs to fix, 
+features to add, or documentation that looks outdated.
+
+## Contributions
+
+This project welcomes contributions from everyone.
+
+Contributions to github-java-client should be made in the form of GitHub pull requests. Each pull 
+request will be reviewed by a maintainer of the library and either merged and released or given 
+feedback for changes that would be required.
+
+## Pull Request Checklist
+
+- Branch from the master branch and, if needed, rebase to the current master branch before 
+  submitting your pull request. If it doesn't merge cleanly with master you may be asked to rebase 
+  your changes.
+- Commits should be as small as possible while ensuring that each commit is valid independently
+  (i.e. each commit should compile and the tests should pass).
+- Add tests relevant to the fixed bug or new feature. We love to increase our test coverage so any 
+  contributions made to improving that will be very welcomed.
+
+## Coding Standards
+
+- This library is modelled after the [GitHub API](https://docs.github.com/en/rest?apiVersion=2022-11-28) and it has been structured to mimic that. 
+  For example, to access the Teams endpoints, you need to instantiate an `OrganisationClient`  
+  and then a `TeamsClient` as [seen here](./src/main/java/com/spotify/github/v3/clients/OrganisationClient.java). This mirrors the nested structure of the API endpoints such as the 
+  [/orgs/{org}/teams/{team_slug}](https://docs.github.com/en/rest/teams/teams?apiVersion=2022-11-28#list-teams) endpoint
+- We operate a monkey see, monkey do approach to this library. We understand that there are some inconsistencies in the library 
+  in terms of how the tests and/or endpoints are written but we, with your help, are working on creating a more consistent codebase.
+- All bug fixes and new features need to be fully tested.