Update note on contributing (#227)

Author: milesfrain

CONTRIBUTING.md

@@ -1 +1,11 @@
-Please make pull requests here!
+Please open PRs or Issues if you find anything to improve in the book.
+
+We appreciate your feedback.
+
+### Editing chapter text:
+
+To test changes to the text locally, install [mdbook](https://github.com/rust-lang/mdBook), then run this command from repo root:
+```
+mdbook serve -o
+```
+The rendered webpage will automatically refresh with any changes to readme files.