[blimp] Add link to Gitiles Markdown reference.

Markdown is a new concept for many chromium engineers, and as such
we want to make it easy for contributors to find out how to use it.

This CL adds a new page for learning more about Markdown. The
page links to the official documentation for Gitiles Markdown, which
is the version used to render Chromium documentation, and also
provides instructions for how to get a local preview of changes while
adding or updating our current Markdown pages.

BUG=None

Review URL: https://codereview.chromium.org/1414453002

Cr-Commit-Position: refs/heads/master@{#354588}

1 files changed, 25 insertions, 0 deletions

diff --git a/blimp/docs/markdown.md b/blimp/docs/markdown.md
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+# How to use Markdown
+
+Markdown is a lightweight markup language which can be easily converted to
+HTML.
+
+## Gitiles reference guide
+
+For the full reference guide for the version of Markdown used to render Chromium
+pages, see the [reference for Gitiles Markdown][gtref].
+
+## Local preview of changes
+
+To see how the pages you edit will look, run the following command from your
+Chromium checkout (`src/` directory) to start up a server on
+http://localhost:8080/:
+
+```bash
+python tools/md_browser/md_browser.py
+```
+
+You can then navigate to http://localhost:8080/blimp/README.md
+in your browser to see the result locally. Just refresh the page after saving
+a `*.md` file to see an updated result.
+
+[gtref]: https://gerrit.googlesource.com/gitiles/+/master/Documentation/markdown.md