Add README for parser directory

src/backend/parser/README

+This directory does more than tokenize and parse SQL queries.  It also
+creates Query structures for the various complex queries that is passed
+to the optimizer and then executor.
+
+parser.c	things start here
+scan.l		break query into tokens
+scansup.c	handle escapes in input
+keywords.c	turn keywords into specific tokens
+gram.y		parse the tokens and fill query-type-specific structures
+analyze.c	one function for every complex query type like SELECT, INSERT...
+parse_clause.c	handle clauses like WHERE, ORDER BY, GROUP BY, ...
+parse_expr.c	handle expressions like col, col + 3, x = 3 or x = 4
+parse_oper.c	handle operations in expressions
+parse_agg.c	handle aggregates, like SUM(col1),  AVG(col2), ...
+parse_func.c	handle functions like date_part(col3), trim(col4), ...
+
+parse_node.c	create nodes for various structures
+parse_target.c	handle the result list of the query
+parse_relation.c support routines for tables and column handling
+parse_type.c	support routines for type handling