[release-branch.go1.10] all: merge master into release-branch.go1.10

* 74b56022 doc: note that x509 cert parsing rejects some more certs now
* c52e27e6 CONTRIBUTING: remove Pull Request bit
* 829b64c1 cmd/fix: fix cast check
* ee59f6df doc: minor wording improvement to the diagnostics guide
* c6e7330e all: remove PULL_REQUEST_TEMPLATE from .github
* d814c2be doc: remove Pull Request note in README.md
* 104445e3 doc: document Go 1.9.4 and Go 1.8.7

Change-Id: I58bfc6800964504258690d774a9b0aeaba509086