Document 3rd party extensions

[mgood]

There should be documentation on known 3rd party extensions, along with guidelines on contributing new ones. Docs should cover how to contribute new panels to the list, as well as some information on how to write a new panel.

There are a couple existing 3rd party add-on panels like: https://pypi.python.org/pypi/Flask-DebugToolbar-Mongo https://pypi.python.org/pypi/Flask-DebugToolbar-LineProfilerPanel

[jaredvacanti]

If some of the 'standards' are always included, DEBUG_TB_PANELS should still include the defaults and the list is for additional panels.

[kevinbeaty]

If anyone is interested, I've created Flask-Debug-API allows browsing and debugging AJAX endpoints

[quantus]

I wanted to see flamegraph visualization for the requests so I ported djdt-flamegraph: https://pypi.python.org/pypi/flask-debugtoolbar-flamegraph