[Feat] add contributing and more build instructions

Also tune up build.gradle for handling non existing key.properties file:
Simplify local dev with no signing keys.

CONTRIBUTING.md

@@ -0,0 +1,68 @@
+# Contributing
+
+Contributions are welcome, and they are greatly appreciated\! Every
+little bit helps, and credit will always be given.
+
+You can contribute in many ways:
+
+## Types of Contributions
+
+### Report Bugs
+
+Report bugs at <https://github.com/GitJournal/GitJournal/issues>.
+
+If you are reporting a bug, please include:
+
+  - Your operating system name and version.
+  - Any details about your local setup that might be helpful in
+    troubleshooting.
+  - Detailed steps to reproduce the bug.
+
+### Fix Bugs
+
+Look through the GitHub issues for bugs. Anything tagged with "bug" is
+open to whoever wants to implement it.
+
+### Implement Features
+
+Look through the GitHub issues for features. Anything tagged with
+"feature" is open to whoever wants to implement it.
+
+### Submit Feedback
+
+The best way to send feedback is to file an issue at
+<https://github.com/GitJournal/GitJournal/issues>.
+
+If you are proposing a feature:
+
+  - Explain in detail how it would work.
+  - Keep the scope as narrow as possible, to make it easier to
+    implement.
+  - Remember that this is a volunteer-driven project, and that
+    contributions are welcome :)
+
+## Get Started!
+
+Ready to contribute? Here's how to set up GitJournal for local development.
+
+1.  Fork the GitJournal repo on GitHub.
+
+2.  Clone your fork locally:
+    
+        $ git clone git@github.com:your_name_here/GitJournal.git
+
+3.  Refer to [build instructions](BUILD.md) for local development.
+
+4.  Create a branch for local development:
+    
+        $ git checkout -b name-of-your-bugfix-or-feature
+    
+    Now you can make your changes locally.
+
+5.  Commit your changes and push your branch to GitHub:
+    
+        $ git add .
+        $ git commit -m "Your detailed description of your changes."
+        $ git push origin name-of-your-bugfix-or-feature
+
+6.  Submit a pull request through the GitHub website.