Add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,50 @@
+# How to contribute
+
+Thank you for your interest in contributing to MARTINS.js! 👍
+
+In order to contribute to MARTINS.js, you must certify that you have the right to submit your contribution according to the [Developer Certificate of Origin (DCO)](https://developercertificate.org). Each commit must include a `Signed-off-by` line at the end of the commit message, as in the template below:
+
+```
+Signed-off-by: Your Name <your.email@addr.es>
+```
+
+This can easily be done with `git commit -s` or `git commit --signoff`. You must use your real name and include a valid e-mail address of yours.
+
+By including a `Signed-off-by` line, you agree to and certify the following:
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+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.
+```