Adding LICENSE file for Affero GPL

pelican-plugins does not have a LICENSE file. The list of contributors is relatively small at this point, so it's probably best to resolve this sooner than later. Current contributors, not including open pull requests:

* ametaireau
* barrysteyn
* avaris
* talha131 -- `goodreads_activity` plugin is "Copyright (c) Talha Mansoor"
* mbustamanteo
* justinmayer
* Lothiraldan -- `multi_part` plugin is "Copyright (c) FELD Boris \<lothiraldan@gmail.com>"
* m-r-r -- `sitemap` was imported from the AGPL getpelican/pelican repository

It would be relatively easy to ping these people via GitHub mentions in this pull request, soliciting consent to the more explicit licensing.