Document locking selects

CHANGES.md

@@ -1,5 +1,6 @@
 ## 0.5.3 In development
 
+* Support locking selects (@dball)
 * Add sql array type and reader literal (@loganmhb)
 
 ## 0.5.2

README.md

@@ -215,6 +215,21 @@ To quote identifiers, pass the `:quoting` keyword option to `format`. Valid opti
 => ["SELECT `foo`.`a` FROM `foo` WHERE `foo`.`a` = ?" "baz"]
 ```
 
+To issue a locking select, add a :lock to the query or use the lock helper. The lock value must be a map with a :mode value. The built-in
+modes are the standard :update (FOR UPDATE) or the vendor-specific :mysql-share (LOCK IN SHARE MODE) or :postresql-share (FOR SHARE). The
+lock map may also provide a :wait value, which if false will append the NOWAIT parameter, supported by PostgreSQL.
+
+```clj
+(-> (select :foo.a)
+    (from :foo)
+    (where [:= foo.a "baz"])
+    (lock {:mode :update})
+    (sql/format))
+=> ["SELECT foo.a FROM foo WHERE foo.a = ? FOR UPDATE" "baz"]
+```
+
+To support novel lock modes, implement the `format-lock-clause` multimethod.
+
 Here's a big, complicated query. Note that Honey SQL makes no attempt to verify that your queries make any sense. It merely renders surface syntax.
 
 ```clj