This is to get the colourspace information commit in,
 but it also performs a bit of cleanup regarding the entry in the thirdparty README.

The reason libwebm wasn't synced is because it has a bunch of unmarked changes,
 and it'd be better if the person responsible untangled that as
 they may know what they did and why they did it.

Given this, it might be a good idea to disconnect libwebm from the libsimplewebm code.