Add contributing guideline

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+## Your new git-extra command should support...
+
+* Should support Mac and Linux(You may need to browse their man page)
+* Should support Bash 3 or newer(If you are not sure, [Bash versions](http://tldp.org/LDP/abs/html/bash2.html))
+* Should support Git 2.1+
+
+## 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 is done.