Update compression documentation

Add "EXPERIMENTAL" to the compress section header.  This change is intended to draw more attention to the status of the feature as stated in the first sentence of the section.  This is not a change in the status.

Author: garyburd

doc.go

@@ -150,7 +150,7 @@
 // application's responsibility to check the Origin header before calling
 // Upgrade.
 //
-// Compression
+// Compression EXPERIMENTAL
 //
 // Per message compression extensions (RFC 7692) are experimentally supported
 // by this package in a limited capacity. Setting the EnableCompression option