Add a proper README on how to use the Pandoc rule.

README.md

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+# Bazel rules for Pandoc
+
+This repository provides a function for
+[the Bazel build system](https://bazel.build/) called `pandoc()` that
+invokes [the Pandoc document converter](https://pandoc.org/). Example
+use cases include converting documentation written in Markdown to HTML
+files, or embedding them as chapters into LaTeX documents. These rules
+depend on the official release binaries provided by the Pandoc project.
+
+# Using these rules
+
+Add the following to your `WORKSPACE` file:
+
+```python
+http_archive(
+    name = "bazel_pandoc",
+    sha256 = "<checksum>",
+    strip_prefix = "bazel-pandoc-<release>",
+    url = "https://github.com/ProdriveTechnologies/bazel-pandoc/archive/v<release>.tar.gz",
+)
+
+load("@bazel_pandoc//:repositories.bzl", "pandoc_repositories")
+
+pandoc_repositories()
+```
+
+You can then add directives along these lines to your `BUILD.bazel` files:
+
+```python
+load("@bazel_pandoc//:pandoc.bzl", "pandoc")
+
+pandoc(
+    name = "foo",
+    src = "foo.md",
+    from_format = "markdown",
+    to_format = "latex",
+)
+```
+
+In the example above, an output file called `foo.tex` will be created in
+the `bazel-bin` directory. The `to_format` field is used to
+automatically derive a file extension of the output file.