Add CONTRIBUTING.md

Based on https://github.com/SELinuxProject/selinux/wiki/Contributing

Signed-off-by: Petr Lautrbach <plautrba@redhat.com>
Acked-by: Paul Moore <paul@paul-moore.com>

CONTRIBUTING.md

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+# Contributing to SELinux
+
+Contributing to the SELinux userspace project is a similar process to
+other open source projects. Bug reports, new features to the existing
+code, additional tools, or updated documentation are all welcome.
+
+You can find a list of open issues where you might contribute to the SELinux kernel code at
+https://github.com/SELinuxProject/selinux-kernel/issues or to the SELinux userspace code at
+https://github.com/SELinuxProject/selinux/issues.
+
+See the selinuxproject.org [user resources
+page](http://selinuxproject.org/page/User_Resources) for more
+information on mailing lists, documentation, and other resources.
+
+## Reporting Bugs
+
+All bugs and patches should be submitted to the [SELinux mailing
+list](https://lore.kernel.org/selinux) at selinux@vger.kernel.org.
+
+When reporting bugs please include versions of SELinux related libraries and
+tools (libsepol, libselinux, libsemanage, checkpolicy). If you are
+using a custom policy please include it as well.
+
+## Compiling
+
+There are a number of dependencies required to build the userspace
+tools/libraries. On a Fedora system you can install them with yum:
+
+    # yum install audit-libs-devel bison bzip2-devel dbus-devel dbus-glib-devel flex flex-devel flex-static glib2-devel libcap-devel libcap-ng-devel pam-devel pcre-devel python-devel setools-devel swig ustr-devel
+
+The tools and libraries can be built and installed under a private directory from the top level with make, e.g.
+
+    $ make DESTDIR=~/obj install install-pywrap
+
+## Contributing Code
+
+After obtaining the code of the repository (see below), create a patch
+against the repository, and post that patch to the [SELinux mailing
+list](https://lore.kernel.org/selinux) at selinux@vger.kernel.org. When preparing
+patches, please follow these guidelines:
+
+-   Patches should apply with -p1
+-   Must apply against HEAD of the master branch
+-   Separate large patches into logical patches
+-   Patch descriptions must end with your "Signed-off-by" line. This means your
+    code meets the Developer's certificate of origin, see bellow.
+
+When adding new, large features or tools it is best to discuss the
+design on the mailing list prior to submitting the patch.
+
+## Development Repository
+
+Git is a modern source code management system. For more information
+about Git please see the Git website.
+
+To get an anonymous checkout of the SELinux userland repository you can
+run:
+
+    $ git clone https://github.com/SELinuxProject/selinux.git
+
+# Developer Certificate of Origin
+
+    Developer's Certificate of Origin 1.1
+
+    By making a contribution to this project, I certify that:
+
+    (a) The contribution was created in whole or in part by me and I
+        have the right to submit it under the open source license
+        indicated in the file; or
+
+    (b) The contribution is based upon previous work that, to the best
+        of my knowledge, is covered under an appropriate open source
+        license and I have the right under that license to submit that
+        work with modifications, whether created in whole or in part
+        by me, under the same open source license (unless I am
+        permitted to submit under a different license), as indicated
+        in the file; or
+
+    (c) The contribution was provided directly to me by some other
+        person who certified (a), (b) or (c) and I have not modified
+        it.
+
+    (d) I understand and agree that this project and the contribution
+        are public and that a record of the contribution (including all
+        personal information I submit with it, including my sign-off) is
+        maintained indefinitely and may be redistributed consistent with
+        this project or the open source license(s) involved.