Revise link to issues for contributing (#4145)

* Revise link to issues for contributing
We are now using the label `good first issue` instead of
`difficulty:Easy` as that is what Github is promoting more.

* Revisit link to starter issues in CONTRIBUTING guidelines as well.

CONTRIBUTING.md

@@ -17,7 +17,7 @@ for support - use [the 'wagtail' tag on Stack Overflow](http://stackoverflow.com
 Please review the 
 [contributing guidelines](http://docs.wagtail.io/en/latest/contributing/index.html). 
 You might like to start by checking issues with the 
-[difficulty:Easy](https://github.com/wagtail/wagtail/labels/difficulty%3AEasy) label.
+[good first issue](https://github.com/wagtail/wagtail/labels/good%20first%20issue) label.
 
 ## Code reviews
 

README.rst

@@ -86,6 +86,6 @@ Contributing
 ~~~~~~~~~~~~
 If you're a Python or Django developer, fork the repo and get stuck in! We run a separate group for developers of Wagtail itself at https://groups.google.com/forum/#!forum/wagtail-developers (please note that this is not for support requests).
 
-You might like to start by reviewing the `contributing guidelines <http://docs.wagtail.io/en/latest/contributing/index.html>`_ and checking issues with the `difficulty:Easy <https://github.com/wagtail/wagtail/labels/difficulty%3AEasy>`_ label.
+You might like to start by reviewing the `contributing guidelines <http://docs.wagtail.io/en/latest/contributing/index.html>`_ and checking issues with the `good first issue <https://github.com/wagtail/wagtail/labels/good%20first%20issue>`_ label.
 
 We also welcome translations for Wagtail's interface. Translation work should be submitted through `Transifex <https://www.transifex.com/projects/p/wagtail/>`_.