Merge pull request #555 from spacewander/contributing

Add Contributing guideline

CONTRIBUTING.md

@@ -0,0 +1,21 @@
+## Your new git-extra command should support
+
+* OSX, Linux, BSD (You may need to browse their man page)<sup>*</sup>
+* Bash 3.2+ (If you aren't sure, see [the Bash changelog](http://tldp.org/LDP/abs/html/bash2.html))
+* Git 2.1+
+
+<sup>*</sup>If you aren't able to test your new command on a platform, 
+make that clear in your PR and someone else may be able to test it on their system.
+
+## To submit a new command, you should
+
+Let's assume your new command is named `foo`.
+
+1. Write a bash script under `./bin` called `git-foo`.
+2. Read `./man/Readme.md` and write documentation for `git-foo`.
+3. Don't forget to introduce it in `Commands.md`.
+4. Update `./etc/git-extras-completion.zsh`. Just follow existing code.
+5. (Optional) Update `./etc/bash_completion.sh`.
+6. Run `./check_integrity.sh foo` to check if all done.
+
+You are welcome to open up an issue to discuss new commands or features before opening a pull request.