MXS-591 Add filter for injecting comments

Comments specified in the cnf file will be injected as a comment before
every sql statement.

Documentation/Filters/Comment.md

@@ -0,0 +1,70 @@
+
+# Comment Filter
+
+## Overview
+
+With the _comment_ filter it is possible to define comments that are
+injected before the actual statements. These comments appear as sql
+comments when they are received by the server.
+
+```
+[MyComment]
+type=filter
+module=comment
+inject=Comment to be injected
+
+[MyService]
+type=service
+router=readwritesplit
+servers=server1
+user=myuser
+passwd=mypasswd
+filters=MyComment
+```
+
+
+## Filter Parameters
+
+The Comment filter requires one mandatory parameter to be defined.
+
+### `inject`
+
+A parameter that contains the comment injected before the statements.
+There is also defined variable $IP that can be used to comment the
+IP address of the client in the injected comment.
+Variables must be written in all caps.
+
+
+## Examples
+
+### Example 1 - Inject IP address of the connected client into statements
+as comment.
+
+You want to see the IP addresses of the clients with the server so you
+define filter to comment the into the statements they send.
+
+
+```
+[IPComment]
+type=filter
+module=comment
+inject=IP=$IP
+
+[MyService]
+type=service
+router=readwritesplit
+servers=server1
+user=myuser
+passwd=mypasswd
+filters=IPComment
+```
+
+In this example when MaxScale receives statement like:
+```
+ SELECT user FROM people;
+```
+It would look like
+```
+/* IP=::ffff:127.0.0.1 */SELECT user FROM people;
+```
+when received by server.