Add contributing docs

CONTRIBUTING.md

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+# Filing Issues
+A good issue can mean the difference between a quick fix and a long, painful fixing process. That's why the
+following guidelines exist:
+
+ - Use the [Github issue tracker](https://github.com/matrix-org/matrix-appservice-bridge/issues) to file your issues.
+ - Write a short title which neatly summaries the *problem*. Do **not** write the *solution* in the issue title.
+   For example: `Cannot create a nick with | in it` is a good issue title. `Filter nicks according to RFC 2812`
+   is not a good issue title.
+ - Give a summary and as much information (along with proposed solutions) as possible in the body of the issue.
+ - Include reproduction steps where possible.
+ - Provide the commit SHA or version number of the `matrix-appservice-bridge` being used.
+
+# Making Pull Requests
+This project follows "git flow" semantics. In practice, this means:
+ - The `master` branch is latest current stable release.
+ - The `develop` branch is where all the new code ends up.
+ - When forking the project, fork from `develop` and then write your code.
+ - Make sure your new code passes all the code checks (tests and linting). Do this by running
+   `npm run test && npm run lint`.
+ - Create a pull request. If this PR fixes an issue, link to it by referring to its number.
+ - PRs from community members must be signed off as per Synapse's [Sign off section](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#sign-off)
+ - Create a changelog entry in `changelog.d`. A changelog filename should be `${GithubPRNumber}.{bugfix|misc|feature|doc|removal}`
+   The change should include information that is useful to the user rather than the developer.
+   You can choose to sign your changelog entry to be credited by appending something like "Thanks to @Half-Shot"
+   at the end of the file, on the same line.

changelog.d/269.doc

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+Add contributing docs to make first time contributions easier