[CORE] Add ActivityPub plugin

This is not the same as the one in https://notabug.org/diogo/gnu-social-activitypub-plugin
Differences to the first "release"
-> Doesn't use guzzle nor has any composer dependencies
-> Supports HTTP Signatures
-> Has basic l10n/i18n
-> Some minor bug fixes

plugins/ActivityPub/README.md

@@ -0,0 +1,42 @@
+# ActivityPub plugin for GNU social
+(c) 2018-2019 Free Software Foundation, Inc
+
+This is the README file for GNU social's ActivityPub plugin.
+It includes general information about the plugin.
+
+## About
+
+This plugin adds [ActivityPub](https://www.w3.org/TR/activitypub/) support to GNU social.
+
+## Credits
+
+* **[Diogo Cordeiro](https://www.diogo.site/)**
+
+## Special thanks
+
+* **[Daniel Supernault](https://github.com/dansup)**
+* **[Mikael Nordfeldth](https://mmn-o.se/)**
+
+## License
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, either version 3 of the
+License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public
+License along with this program, in the file "COPYING".  If not, see
+<http://www.gnu.org/licenses/>.
+
+    IMPORTANT NOTE: The GNU Affero General Public License (AGPL) has
+    *different requirements* from the "regular" GPL. In particular, if
+    you make modifications to the plugin source code on your server,
+    you *MUST MAKE AVAILABLE* the modified version of the source code
+    to your users under the same license. This is a legal requirement
+    of using the software, and if you do not wish to share your
+    modifications, *YOU MAY NOT USE THIS PLUGIN*.