Add Travis info to Drill contribution doc page

Add the following content from Timothy to the https://drill.apache.org/docs/contribute-to-drill/ page: Travis has been configured to run smoke tests every time PRs are opened or updated now. Travis was broken before the last batch commit, but the latest code in master successfully passes tests on Travis. When you open PRs please make sure your tests pass on Travis (there is a green check next to your PR). If there is a failure you should be able to click on the red X next to your PR and check the Travis logs to debug the issue. Also if you link your github account to travis-ci.org you will be able to have Travis tests for Drill run on your own private branches whenever you push a commit. Incase your wondering, what is a smoke test? A smoke test is any test that is not a SlowTest, UnlikelyTest, or SecurityTest. How do you tell if a test is a SlowTest, UnlikelyTest, SecurityTest? Just check to see if there is an @SlowTest, @UnlikelyTest, or @SecurityTest annotation at the top of the test class. What is a SlowTest? It's any test that is very heavy weight and takes a long time to run. What is a SecurityTest? Any test which checks a security related feature. What is an UnlikelyTest? A test which checks something very esoteric like a UDF which very few people will modify. Also please keep in mind whether your unit test needs to run on Travis and tag it as a SlowTest, UnlikelyTest, or SecurityTest appropriately if it doesn't need to run on Travis. Travis has limited resources so it cannot run all of the unit tests.