Add guidelines to add this list in third-party projects. Fix #31 (#35)

* Add guidelines to add this list in third-party projects. Fix #31

A stab at increasing the audience of this list by featuring it in Wagtail projects. 

Addresses #31.

* Tweak Markdown markup slightly

CONTRIBUTING.md

@@ -22,6 +22,15 @@ Thank you for your suggestions!
 
 Once you submit the pull request, the links will be checked by [awesome_bot](https://github.com/dkhamsing/awesome_bot) via [our Travis build](https://travis-ci.org/springload/awesome-wagtail).
 
+### Spreading the word
+
+If you want to help spread the word about this list, consider adding a mention of it in your project's README: `*Check out [Awesome Wagtail](https://github.com/springload/awesome-wagtail) for more awesome packages and resources from the Wagtail community.*`
+
 ### Updating your PR
 
 A lot of times, making a PR adhere to the standards above can be difficult. If the maintainers notice anything that we'd like changed, we'll ask you to edit your PR before we merge it. If you're not sure how to do that, [here is a guide](https://github.com/RichardLitt/docs/blob/master/amending-a-commit-guide.md) on the different ways you can update your PR so that we can merge it.
+
+### Accepting a PR
+
+- Ensure the pull request adheres to the [contribution guidelines](#contribution-guidelines).
+- When merging the PR, consider [suggesting the addition of the list](#spreading-the-word).