Add a README

README.md

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+# Bazel build rules for GNU M4
+
+## Overview
+
+```python
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
+    name = "rules_m4",
+    # See https://github.com/jmillikin/rules_m4/releases for copy-pastable
+    # URLs and checksums.
+)
+
+load("@rules_m4//m4:m4.bzl", "m4_register_toolchains")
+
+m4_register_toolchains()
+```
+
+```python
+load("@rules_m4//m4:m4.bzl", "m4")
+
+m4(
+    name = "hello_world",
+    srcs = ["hello_world.in.txt"],
+    output = "hello_world.txt",
+)
+```
+
+```python
+genrule(
+    name = "hello_world_gen",
+    srcs = ["hello_world.in.txt"],
+    outs = ["hello_world_gen.txt"],
+    cmd = "$(M4) $(SRCS) > $@",
+    toolchains = ["@rules_m4//m4:toolchain"],
+)
+```
+
+## Toolchains
+
+```python
+load("@rules_m4//m4:m4.bzl", "m4_common")
+
+def _my_rule(ctx):
+    m4_toolchain = m4_common.m4_toolchain(ctx)
+    ctx.actions.run(
+        executable = m4_toolchain.m4_executable,
+        inputs = m4_toolchain.files,
+        # ...
+    )
+
+my_rule = rule(
+    _my_rule,
+    toolchains = [m4_common.TOOLCHAIN_TYPE],
+)
+```