Add start of contribution information

HOW-TO-CONTRIBUTE.md

+Development is handled through GitHub
+
+All code changes must be committed via git to a local fork
+and contributed back to the project via a pull request.
+
+Ideally each developer should have a fork of the project
+on GitHub where they can push changes.
+
+In your local clone:
+
+ * git pull origin develop
+ * git checkout -b topics/whatever-you-work-on (or bugfix/NUM — for bugs)
+ * write code and commit
+ * git push origin topics/whatever-you-work-on
+ * go on github.com/github.unl.edu and open a pull request from your branch to develop
+ * have someone else review
+
+Another developer will review your changes and merge in to the develop branch.