Add module command documetation

Added a document that describes the module command system and added the
necessary information in the dbfwfilter documentation.

The release notes also point to the newly created document.

Documentation/Reference/Module-Commands.md

@@ -0,0 +1,65 @@
+# Module commands
+
+Introduced in MaxScale 2.1, the module commands are special, module-specific
+commands. They allow the modules to expand beyond the capabilities of the
+module API. Currently, only MaxAdmin implements an interface to the module
+commands.
+
+All registered module commands can be shown with `maxadmin list functions` and
+they can be executed with `maxadmin call function <domain> <name> ARGS...` where
+_<domain>_ is the domain where the module registered the function and _<name>_
+is the name of the function. _ARGS_ is a function specific list of arguments.
+
+## Developer reference
+
+The module command API is defined in the _modulecmd.h_ header. It consists of
+various functions to register and call module commands. Read the function
+documentation in the header for more details.
+
+The following example registers the module command _my_command_ in the _my_module_ domain.
+
+```
+#include <maxscale/modulecmd.h>
+
+bool my_simple_cmd(const MODULECMD_ARG *argv)
+{
+    printf("%d arguments given\n", argv->argc);
+}
+
+int main(int argc, char **argv)
+{
+    modulecmd_arg_type_t my_args[] =
+    {
+        {MODULECMD_ARG_BOOLEAN, "This is a boolean parameter"},
+        {MODULECMD_ARG_STRING | MODULECMD_ARG_OPTIONAL, "This is an optional string parameter"}
+    };
+
+    // Register the command
+    modulecmd_register_command("my_module", "my_command", my_simple_cmd, 2, my_args);
+
+    // Find the registered command
+    const MODULECMD *cmd = modulecmd_find_command("my_module", "my_command");
+
+    // Parse the arguments for the command
+    const void *arglist[] = {"true", "optional string"};
+    MODULECMD_ARG *arg = modulecmd_arg_parse(cmd, arglist, 2);
+
+    // Call the module command
+    modulecmd_call_command(cmd, arg);
+
+    // Free the parsed arguments
+    modulecmd_arg_free(arg);
+    return 0;
+}
+```
+
+The array of _modulecmd_arg_type_t_ type is used to tell what kinds of arguments
+the command expects. The first argument is a SERVER which will be replaced with a
+pointer to a server. The second argument is an optional string argument.
+
+Arguments are passed to the parsing function as an array of void pointers. They
+are interpreted as the types the command expects.
+
+When the module command is executed, the _argv_ parameter for the
+_my_simple_cmd_ contains the parsed arguments received from the caller of the
+command.