Add CONTRIBUTING file

CONTRIBUTING

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+All contributors to Cert Spotter must accept the Developer Certificate
+of Origin (DCO):
+
+	Developer Certificate of Origin
+	Version 1.1
+
+	Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+	660 York Street, Suite 102,
+	San Francisco, CA 94110 USA
+
+	Everyone is permitted to copy and distribute verbatim copies of this
+	license document, but changing it is not allowed.
+
+
+	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.
+
+To accept the DCO, add the following line to each commit message with
+your real name and email address (`git commit -s` will do this for you):
+
+	Signed-off-by: Alex Smith <alex.smith@example.com>