doc: write release notes

[rsc]

Go 1.7 beta needs proper release notes. They are in progress.

[dsnet]

Slight change in compress/flate in that it now returns io.EOF eagerly. This may break some tests that incorrectly assumed flate never did that.

[gopherbot]

CL https://golang.org/cl/23493 mentions this issue.

[gopherbot]

CL https://golang.org/cl/23488 mentions this issue.

[kostya-sh]

https://tip.golang.org/doc/go1.7 still mentions SizedReaderAt and File.Size

Also new package net/http/httptrace (https://tip.golang.org/pkg/net/http/httptrace/) is not mentioned.

[rsc]

Leaving these last few fixes for @adg. Thanks for catching those @kostya-sh. For that matter api/next.txt needs updating too.

[rsc]

@adg, in addition to what @kostya-sh said, see @dsnet's comment above.

[adg]

I believe the docs are in good shape for the beta now.