Note on test cases

Author: bas-ie

CONTRIBUTING.md

@@ -2,6 +2,6 @@ Thanks for helping us out! We really appreciate it.
 
 Please make sure you're working against a current copy of the `master` branch. Because `react-native-htmlview` is a small package, with a lot of people working on the same files your changes can go 'stale' quite quickly. Please don't be offended if we ask you to resolve conflicts on a PR that's sat unreviewed for a little while.
 
-Please run `yarn test` (which will invoke the linter) prior to submitting your PR. If the linter finds any problems, you can usually fix with `yarn run format` and commit the result.
+Please run `yarn test` (which will invoke the linter) prior to submitting your PR. If the linter finds any problems, you can usually fix with `yarn run format` and commit the result. If you include a new feature, add a test case for it (we mostly use snapshot testing).
 
 Finally, please provide enough detail with your PR for us to be able to see what you intend, or the problem you're trying to fix. Screenshots are awesome if possible.