Add basic documentation

Add an "About" page with a description of what the plugin does, copied
from core Gerrit's plugin documentation [1].

[1] https://gerrit-review.googlesource.com/Documentation/config-plugins.html#singleusergroup

Change-Id: I959195881bf3dd53d40b00eabd5b2e4ffd94e2b7

BUCK

diff --git a/BUCK b/BUCK
index b7356f2..0e2621e 100644
--- a/BUCK
+++ b/BUCK
@@ -1,6 +1,7 @@
 gerrit_plugin(
   name = 'singleusergroup',
   srcs = glob(['src/main/java/**/*.java']),
+  resources = glob(['src/main/resources/**/*']),
   manifest_entries = [
     'Gerrit-PluginName: singleusergroup',
     'Gerrit-Module: com.googlesource.gerrit.plugins.singleusergroup.SingleUserGroup$Module',

src/main/resources/Documentation/about.md

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
new file mode 100644
index 0000000..86d4d17
--- /dev/null
+++ b/src/main/resources/Documentation/about.md
@@ -0,0 +1,3 @@
+This plugin provides a group per user. This is useful to assign access
+rights directly to a single user, since in Gerrit access rights can
+only be assigned to groups.