Convert the contents of the top-level README to Markdown and give it a
.md extension. Most of our documentation is plain text or AsciiDoc, but
the top-level README file in a Git repository is special in that many
online browsers will show the README contents along with the directory
listing and those browsers tend to favor Markdown. This is true of
GitHub (which we're currently mirroring to), Gerrit via its Gitiles
plugin (which we're not yet using but likely will), and other places.

Add "foreign" to AM_INIT_AUTOMAKE. There is probably a joke to be
made here about the FSF and border walls.

Change-Id: I87c306d74864e1f0a432225b160a1b4483ee946c
Reviewed-on: https://code.wireshark.org/review/23049


Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>