Add a CONTRIBUTING file for Github

CONTRIBUTING.md

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+This file serves as a supplement to the [HACKING file](HACKING). It contains
+information specifically about Nmap's use of Github and how contributors can use
+Github services to participate in Nmap development.
+
+## Code Repository
+
+The authoritative code repository is still the Subversion repository at
+https://svn.nmap.org/nmap . The Github repository is synchronized once per hour.
+All commits are made directly to Subversion, so Github is a read-only mirror.
+
+## Bug Reports
+
+Nmap uses Github Issues to keep track of bug reports. Please be sure to include
+the version of Nmap that you are using, steps to reproduce the bug, and a
+description of what you expect to be the correct behavior.
+
+## Pull Requests
+
+Nmap welcomes your code contribution in the form of a Github Pull Request. Since
+the Github repository is currently read-only, we cannot merge directly from the
+PR. We will be sure to properly credit you in the CHANGELOG file, and the commit
+message will reference the PR number.
+
+Because not all Nmap committers use Github daily, it is helpful to send a
+notification email to dev@nmap.org referencing the PR and including a short
+description of the functionality of the patch.
+
+Using pull requests has several advantages over emailed patches:
+
+1. It allows Travis CI build tests to run and check for code issues.
+
+2. Github's interface makes it easy to have a threaded discussion of code
+changes.
+
+3. Referencing contributions by PR number is more convenient than tracking by
+seclists.org mail archive URL, especially when the discussion spans more than
+one quarter year.
+
+## issues.nmap.org redirector
+
+For convenience, you may use issues.nmap.org to redirect to issues (bug reports
+and pull requests) by number (e.g. http://issues.nmap.org/34) or to link to the
+new-issue page: http://issues.nmap.org/new
+
+## The HACKING file
+
+General information about hacking Nmap and engaging with our community of
+developers and users can be found in the [HACKING file](HACKING). It describes
+how to get started, licensing, style guidance, and how to use the dev mailing
+list.