proposal: update proposal/README.md to stand alone

[rsc]

The proposal process document at https://golang.org/s/proposal has at least two problems we should fix.

First, it incorporates a few talks by reference, but most people aren't going to read/watch those or take away what we want to take away. It's fine to keep the talk links but we should extract the important messages and say them in the README explicitly.

Second, we introduced new process for Go 2 changes, with two rounds of proposals, in the Go 2 here we come blog post, but this is not mentioned in the README.

[rsc]

Correction: the Go 2 blog post is mentioned in the README but like with the talks the relevant parts should be incorporated into the document explicitly.