While it's not mandatory, extractors are highly encouraged to provide a thumbnail field.
These instructions are overly long as it is. Leave out the _TESTS example; most developers will not need it in their first IE.
There was no information in the README about how to handle multiple tests for a given extractor. This commit adds an explanation of how this is handled. It also adds some syntax highlighting.
This seems to be the part most often overlooked in our README.
Add a longer tutorial that should cover everything needed to start developing IEs. Fixes #2676
Conflicts: youtube_dl/PostProcessor.py
Works on Linux, OSX, and Windows.