add first time only clarification

As per geopython#311 (comment)

CONTRIBUTING.rst

@@ -6,6 +6,11 @@ enhancements/features, etc.).  This document will outline some guidelines on
 contributing to pycsw.  As well, pycsw `community </community.html>`_ is a great place to
 get an idea of how to connect and participate in pycsw community and development.
 
+pycsw has the following modes of contribution:
+
+- GitHub Commit Access
+- GitHub Pull Requests
+
 Contributions and Licensing
 ---------------------------
 
@@ -16,13 +21,13 @@ GitHub Commit Access
 
 - proposals to provide developers with GitHub commit access shall be emailed to the pycsw-devel `mailing list`_.  Proposals shall be approved by the pycsw development team.  Committers shall be added by the project admin
 - removal of commit access shall be handled in the same manner
-- each committer must send an email to the pycsw mailing list agreeing to the license guidelines (see `Contributions and Licensing Agreement Template <#contributions-and-licensing-agreement-template>`_)
+- each committer must send an email to the pycsw mailing list agreeing to the license guidelines (see `Contributions and Licensing Agreement Template <#contributions-and-licensing-agreement-template>`_).  **This is only required once**
 - each committer shall be listed in https://github.com/geopython/pycsw/blob/master/COMMITTERS.txt
 
-Pull Requests
-^^^^^^^^^^^^^
+GitHub Pull Requests
+^^^^^^^^^^^^^^^^^^^^
 
-- pull requests can provide agreement to license guidelines as text in the pull request or via email to the pycsw `mailing list`_  (see `Contributions and Licensing Agreement Template <#contributions-and-licensing-agreement-template>`_)
+- pull requests can provide agreement to license guidelines as text in the pull request or via email to the pycsw `mailing list`_  (see `Contributions and Licensing Agreement Template <#contributions-and-licensing-agreement-template>`_).  **This is only required for a contributor's first pull request.  Subsequent pull requests do not require this step**
 - pull requests may include copyright in the source code header by the contributor if the contribution is significant or the contributor wants to claim copyright on their contribution
 - all contributors shall be listed at https://github.com/geopython/pycsw/graphs/contributors