Document the member parser module.

MODULES.md

@@ -307,6 +307,48 @@ Returns the payments (bank transactions) list of given member record
 
 ### Member Parser
 
+This module exports only one function - the ```load-member-file```
+which loads and parses given file as member file. The specification of
+this file format is in a separate document ```MEMBERS.md```.
+
+The module implements this specification as multi-pass parser with
+important definitions being at the top of the module.
+
+    mandatory-keys
+
+A list of symbols containing keys which must be present in the
+file. If any of them is missing, it is added to the resulting
+structure with the ```#f``` value.
+
+    optional-keys
+
+A list of symbols of keys which the parser recognizes and adds them to
+the resulting data structure.
+
+    ignored-keys
+
+A list of symbols with keys that are parsed, and albeit not added to
+the result they do not generate warnings.
+
+    known-multikeys
+
+A list of keys that can appear multiple times and in the 2nd pass they
+are collected as lists of values.
+
+    start-stop-markers-lookup
+
+A list of start/stop specifications - each key is paired with given
+key and start/stop tag.
+
+    (load-member-file mr)
+
+* ```mr``` - (almost) empty member record structure
+
+It must be possible to get the member file using
+```member-record-input-file``` function. Loads the file as a list of
+lines, processes these lines with 1st and 2nd pass parsers, interprets
+the result using 3rd passes and finalizes the result in the 4th pass.
+
 ### Members Directory
 
 ### Members Base