Docs: Contrib. guide, notes on testing

[ci skip]

CONTRIBUTING.md

@@ -1,11 +1,31 @@
 # Contributing
 
-Thanks for your interest in PaperTrail! We appreciate bug reports, feature
-suggestions, and especially pull requests.
-
-Please do not use github issues to ask usage questions. We simply don't have
-time to answer them all. Instead, please ask usage questions on Stack Overflow.
+Thanks for your interest in PaperTrail!
 
+Ask usage questions on Stack Overflow:
 http://stackoverflow.com/tags/papertrail
 
-Thanks, and happy versioning :)
+**Please do not use github issues to ask usage questions.**
+
+On github, we appreciate bug reports, feature
+suggestions, and especially pull requests.
+
+Thanks, and happy (paper) trails :)
+
+## Development
+
+Run tests with sqlite:
+
+```
+bundle exec rake prepare
+bundle exec rake
+```
+
+Run tests with mysql:
+
+```
+cd test/dummy
+RAILS_ENV=test bundle exec rake db:setup
+cd ../..
+DB=mysql bundle exec rake
+```