additional contrib note (#753)

Author: mikebz

CONTRIBUTING.md

@@ -84,6 +84,9 @@ For each function, its files spread in the follow places:
   example here. There must be a README.md file in each example directory, and it
   should follow the [template][example-template].
 - The `tests/` directory contains additional e2e tests.
+- `master` branch should should contain examples with the `unstable` tag for
+  your function images.  When you release the function version that tag should 
+  have the samples and tests that match the function version.
 
 For golang-based functions, you need to generate some doc related variables from
 the `README.md` by running