[dev.boringcrypto] misc/boring: add go1.10.4b4 and go1.11b4 releases

[dev.boringcrypto] misc/boring: add go1.10.4b4 and go1.11b4 releases

Also, document the fact that we cut releases only from the versioned
branches, and use the correct x/net branch.

Had to build this passing -skip_tests to release because the buildlet
was timing out (see below), but the builders on the dashboard are green.

2018/09/28 19:14:50 linux-amd64: Start.
2018/09/28 19:14:50 linux-amd64: Creating buildlet.
2018/09/28 19:15:28 linux-amd64: Pushing source to buildlet.
2018/09/28 19:15:37 linux-amd64: Writing VERSION file.
2018/09/28 19:15:38 linux-amd64: Cleaning goroot (pre-build).
2018/09/28 19:15:38 linux-amd64: Building.
2018/09/28 19:46:20 Buildlet https://farmer.golang.org:443 failed three heartbeats; final error: timeout waiting for headers
2018/09/28 19:46:20 linux-amd64: Error: Buildlet https://farmer.golang.org:443 failed heartbeat after 10.007631241s; marking dead; err=timeout waiting for headers

Change-Id: I9d982df693075f96d44aa6f163533253c8ae2914
Reviewed-on: https://go-review.googlesource.com/138555
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>