Create README

Author: dillon-giacoppo

README

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+# rules_python_external
+
+Contains Bazel rules to fetch and install Python dependencies from a requirements.txt file.
+
+## Usage
+
+In `requirements.txt`
+```
+cryptography[test, docs]
+boto3
+```
+
+In `WORKSPACE`
+```
+load("@rules_pip//:defs.bzl", "pip_repository")
+
+pip_repository(
+    name = "py_deps",
+    requirements = "//:requirements.txt",
+)
+```
+
+In `BUILD`
+```
+load("@py_deps//:requirements.bzl", "requirement")
+
+py_binary(
+    name = "main",
+    srcs = ["main.py"],
+    deps = [
+        requirement("boto3"),
+    ],
+)
+```