doc: update proposed DCO and CoC

Lifted verbatim from
https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
then `s/Node.js/node-gyp/`.

PR-URL: #1229
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

CONTRIBUTING.md

@@ -0,0 +1,34 @@
+# Contributing to node-gyp
+
+## Code of Conduct
+
+Please read the
+[Code of Conduct](https://github.com/nodejs/TSC/blob/master/CODE_OF_CONDUCT.md)
+which explains the minimum behavior expectations for node-gyp contributors.
+
+<a id="developers-certificate-of-origin"></a>
+## 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.