Expand README and add update instructions

tests/phpunit/data/html5lib-tests/README.md

@@ -1,12 +1,25 @@
-html5lib-tests
-==============
+# html5lib-tests
 
-This test data was taken from:
+This directory contains a third-party test suite used for testing the WordPress HTML API.
 
-https://github.com/html5lib/html5lib-tests
+`html5lib-tests` can be found on GitHub at [html5lib/html5lib-tests](https://github.com/html5lib/html5lib-tests).
 
-The sha was `a9f44960a9fedf265093d22b2aa3c7ca123727b9`.
+The necessary files have been copied to this directory:
 
-## Instructions
+- `AUTHORS.rst`
+- `LICENSE`
+- `README.md`
+- `tree-construction/README.md`
+- `tree-construction/*.dat`
 
-@todo add instructions here.
+The version of these files was taken from the git commit with
+SHA [`a9f44960a9fedf265093d22b2aa3c7ca123727b9`](https://github.com/html5lib/html5lib-tests/commit/a9f44960a9fedf265093d22b2aa3c7ca123727b9).
+
+## Updating
+
+If there have been changes to the html5lib-tests repository, this test suite can be updated. In
+order to update:
+
+1. Check out the latest version of git repository mentioned above.
+1. Copy the files listed above into this directory.
+1. Update the SHA mentioned in this README file with the new html5lib-tests SHA.