Add contributing.md with testing guidelines (runatlantis#65)

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+## Testing
+* place tests under `{package under test}_test` to enforce testing the external interfaces
+* if you need to test internally i.e. access non-exported stuff, call the file `{file under test}_internal_test.go`
+* use `testing_util` for easier-to-read assertions: `import . "github.com/hootsuite/atlantis/testing_util"`
+* don't try to describe the whole test by its function name. Instead use `t.Log` statements:
+```go
+// don't do this
+func TestLockingWhenThereIsAnExistingLockForNewEnv(t *testing.T) {
+    ...
+
+// do this
+func TestLockingExisting(t *testing.T) {
+    	t.Log("if there is an existing lock, lock should...")
+        ...
+       	t.Log("...succeed if the new project has a different path") {
+             // optionally wrap in a block so it's easier to read
+       }
+```
+* each test should have a `t.Log` that describes what the current state is and what should happen (like a behavioural test)