docs: explain how to preview changes

docs/README.md

 # Chromium docs
 
 This directory contains chromium project documentation in [Markdown].
 It is automatically
 [rendered by Gitiles](https://chromium.googlesource.com/chromium/src/+/master/docs/).
 
 ## Style guide
 
 Markdown documents must follow the [style guide].
 
+## Previewing changes
+
+You can preview your local changes using [md_browser]:
+
+```bash
+# in chromium checkout
+python tools/md_browser/md_browser.py

[Jeffrey Yasskin, 2015/12/18 23:54:21]

While this is running, http://localhost:8080/ gives me "I do not know how to
serve /.", and http://localhost:8080/docs/ gives me "I do not know how to serve
/docs/.".

[nodir, 2015/12/19 00:18:15]

I think md_browser should support file listing so it is easier to navigate
(+dpranke). With that, these instructions are enough.

Open http://localhost:8080/docs/README.md

[Jeffrey Yasskin, 2015/12/19 00:24:14]

Once md_browser supports file listing, this lgtm.

+```
+
+To review someone else's changes, apply them locally first:
+
+```bash
+# in chromium checkout
+git cl patch <CL number or URL>
+python tools/md_browser/md_browser.py
+```
+
 [Markdown]: https://gerrit.googlesource.com/gitiles/+/master/Documentation/markdown.md
 [style guide]: https://github.com/google/styleguide/tree/gh-pages/docguide
+[md_browser]: ../tools/md_browser/