docs(compile_pip_requirements): Add note on requirements.txt VC (#1245)

The documentation is currently ambiguous on whether or not to check
requirements.txt into version control. This has raised some confusion in
other projects (e.g., google/gematria#3). This
makes it clear that requirements.txt files produced by this rule should
be checked into version control in an easy to find place to avoid
confusion.

Author: boomanaiden154

docs/pip.md

@@ -42,6 +42,9 @@ def compile_pip_requirements(
     - validate with `bazel test [name]_test`
     - update with   `bazel run [name].update`
 
+    If you are using a version control system, the requirements.txt generated by this rule should
+    be checked into it to ensure that all developers/users have the same dependency versions.
+
     Args:
         name: base name for generated targets, typically "requirements".
         extra_args: passed to pip-compile.