Documentation should note that "{HEAD}" is not a valid ref

[pcn]

I was getting the following error:

$ bazel build
ERROR: error loading package '': Encountered error while reading extension file 'python/pip.bzl': no such package '@io_bazel_rules_python//python': Invalid branch, tag, or commit: Ref {HEAD} can not be resolved
ERROR: error loading package '': Encountered error while reading extension file 'python/pip.bzl': no such package '@io_bazel_rules_python//python': Invalid branch, tag, or commit: Ref {HEAD} can not be resolved

and found pubref/rules_node#15.

[mattmoor]

See linked PR