Add loading of git_repository rule to README (bazel-contrib#152)

Author: danieljanes

README.md

@@ -23,6 +23,8 @@ dependencies typically managed via `pip`.
 Add the following to your `WORKSPACE` file to add the external repositories:
 
 ```python
+load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
+
 git_repository(
     name = "io_bazel_rules_python",
     remote = "https://github.com/bazelbuild/rules_python.git",