Add playground hash instructions to contributing.md

Author: dtolnay

CONTRIBUTING.md

@@ -9,3 +9,9 @@ Here is what I run:
 
 $ node_modules/.bin/gitbook build && rm -rf gitbook styles && mv _book/* .
 -->
+
+If your contribution touches example code inside a <code>```rust</code> code
+block which has a `!PLAYGROUND` on the previous line, the hash on that line
+needs to be updated by pasting the new code into play.rust-lang.org, clicking
+Share, and grabbing the new hash from the generated "Permalink to the
+playground" link.