Add 'about' section in documentation

Move the description of the plugin from the config file
to a new file named "about".

Change-Id: I7cde80706a742368fa990ed4937931857bdf7ea6

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..04b05da
--- /dev/null
+++ b/src/main/resources/Documentation/about.md
@@ -0,0 +1,3 @@
+This plugin checks the length of a commit's commit message
+subject and message body, and reports warnings or errors to
+the git client if the lengths are exceeded.

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 86bc611..92440e7 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -1,10 +1,6 @@
 Commit Message Length Configuration
 ===================================
 
-This plugin checks the length of a commit's commit message
-subject and message body, and reports warnings or errors to
-the git client if the lengths are exceeded.
-
 The maximum lengths of the subject and message body can be
 configured in the standard Gerrit config file `gerrit.config`.