Update README Install Documentation.

[jesusalc]

Just like it says. We need to update the Install docs, with more robust data.

[KiChjang]

Is there anything in particular that you think we should add?

[jesusalc]

If you look at #12356 You can see a very well documented Ubuntu 14.04.04 32bit and 64bit, Windows 7 32bit and 64bit documentation. Which looks very through to me. Also, there seem to be many tickets about "servo not building" or l related. Some of these issues could bring documentation to specific versions and situations. Which can make the documentation more complete.

[nox]

We regularly add missing requirements and steps when there are build failures filed here and we found out there was a missing dependency. This issue itself isn't very actionable because I don't know when it can be considered fixed. Closing it.