gopls: add note to vscode doc about tags
gopls: add note to vscode doc about tags

As prompted [this issue](https://github.com/golang/go/issues/38963) it
is unclear how to setup build tags in gopls in vscode.

vscode has a configuration section specific for go build tags but it is
not honored by gopls which instead requires GOFLAGS environment variable
to be set with the -tags flag.

Change-Id: Ib74a5bca78bf222d73224590ee0344948f020f9f
GitHub-Last-Rev: 37d1f9f625f13b5917f983cdd0b1b2a36968457e
GitHub-Pull-Request: golang/tools#227
Reviewed-on: https://go-review.googlesource.com/c/tools/+/233018
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools