readme update
minor.
Revert WsgiMounter optional root support as .check() call somes consumes generators.

This partially reverts commit b3e2ea8c0e10b56e3a160b1c2d1efb245318dc40.
add yaml to multiprot example
Add custom ctor to YamlDocument protocol to make it possible to pass parameters to yaml's dump/load functions.

Also update yaml docstring.