Update prerequisite instructions (especially for Windows).
Update prerequisite instructions (especially for Windows).

The instructions previously suggested that all steps at
http://www.chromium.org/developers/how-tos/build-instructions-windows
should be performed, which is not true. That page now lists 'fetch chromium'
as part of the steps, which is pointless when building for standalone WebRTC.

I polished the rest of the instructions as well.

louquillio/webrtc-org,webrtc/webrtc-org,webrtc/webrtc-org,louquillio/webrtc-org,samdutton/webrtc-org,Scintillo/webrtc-org,webrtc/webrtc-org,Scintillo/webrtc-org,Scintillo/webrtc-org,samdutton/webrtc-org,louquillio/webrtc-org