Add luafilter documentation

The luafilter documentation describes the mechanics of how MaxScale will
call the lua scripts.

Documentation/Filters/Luafilter.md

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+# Lua Filter
+
+The luafilter is a filter that calls a set of functions in a Lua script. The
+filter is currently a part of the experimental module set.
+
+Read the [Lua language documentation](https://www.lua.org/docs.html) for
+information on how to write Lua scripts.
+
+## Filter Parameters
+
+The luafilter has two parameters. They control which scripts will be called by
+the filter. Both parameters are optional but at least one should be defined. If
+both `global_script` and `session_script` are defined, the entry points in both
+scripts will be called.
+
+### `global_script`
+
+The global Lua script. The parameter value is a path to a readable Lua script
+which will be executed.
+
+This script will always be called with the same global Lua state and it can be
+used to build a global view of the whole service.
+
+### `session_script`
+
+The session level Lua script. The parameter value is a path to a readable Lua
+script which will be executed once for each session.
+
+Each session will have its own Lua state meaning that each session can have a
+unique Lua environment. Use this script to do session specific tasks.
+
+## Lua Script Calling Convention
+
+The entry points for the Lua script expect the following signatures:
+
+  - `nil createInstance()` - global script only, called when MaxScale is started
+
+    - The global script will be loaded in this function and executed once on a
+      global level before calling the createInstance function in the Lua script.
+
+  - `nil newSession()` - new session is created
+
+    - This function first loads the session script and executes in on a global
+      level.  After this, the newSession function in the Lua scripts is called.
+      There is a single C function exported as a global variable for the session
+      script named id_gen. The id_gen function returns an integer that is unique
+      for this service only. This function is only accessible to the session
+      level scripts.
+
+  - `nil closeSession()` - session is closed
+
+    - The `closeSession` function in the Lua scripts will be called.
+
+  - `(nil | bool | string) routeQuery(string)` - query is being routed
+  
+    - The Luafilter calls the `routeQuery` functions of both the session and the
+      global script.  The query is passed as a string parameter to the
+      routeQuery Lua function and the return values of the session specific
+      function, if any were returned, are interpreted. If the first value is
+      bool, it is interpreted as a decision whether to route the query or to
+      send an error packet to the client.  If it is a string, the current query
+      is replaced with the return value and the query will be routed. If nil is
+      returned, the query is routed normally.
+
+  - `nil clientReply()` - reply to a query is being routed
+
+    - This function calls the `clientReply` function of the Lua scripts.
+
+  - `string diagnostic()` - global script only, print diagnostic information
+
+    - This will call the matching `diagnostics` entry point in the Lua script. If
+      the Lua function returns a string, it will be printed to the client.
+
+These functions, if found in the script, will be called whenever a call to the
+matching entry point is made.