More info for CONTRIBUTING.md

[rtoy]

Describe the issue
The document CONTRIBUTING.md shows how to use the HTTP API for bikeshed to preview the changes. This is correct, but we should note that because we use some bikeshed macros, the preview will be missing some stuff. In particular, table for the properties of the node (number of inputs/outputs, channel count/mode, tail-time) is missing as well as the table describing the properties of an AudioParam.

[rtoy]

Oh, and maybe suggest running compile.sh to run bikeshed locally. Of course this only works on Linux and Mac.