Check in a simple pure-python based Markdown previewer.

tools/md_browser/README.md

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+# md_browser
+
+This is a simple tool to render the markdown docs in a chromium checkout
+locally. It is written in Python and uses the Python 'markdown' package,
+which is checked into src/third_party.
+
+md_browser attempts to emulate the flavor of Markdown implemented by
+[Gitiles](https://gerrit.googlesource.com/gitiles/+/master/Documentation/markdown.md).
+
+Gitiles is the source browser running on https://chromium.googlesource.com,
+and can be run locally, but to do so requires a Java install and a Buck
+install, which can be slightly annoying to set up on Mac or Windows.
+
+This is a lighterweight solution, which also allows you to preview uncommitted
+changes (i.e., it just serves files out of the filesystem, and is not a
+full Git repo browser like Gitiles is).
+
+To run md_browser:
+
+1. cd to the top of your chromium checkout
+
+2. run `python tools/md_browser/md_browser.py`
+
+3. There is no step three.
+
+This will run a local web server on port 8080 that points to the top
+of the repo.  You can specify a different port with the `-p` flag.