documentation: add description of conn:close() and pool:close() methods

Author: LeonidVas

README.md

@@ -170,6 +170,13 @@ Quote a query string.
  - `quoted_string` on success
  - `error(reason)` on error
 
+### `conn:close()`
+
+Close the individual connection or return it to a pool.
+Throws an error on failure.
+
+*Returns*: `true`
+
 ### `pool = mysql.pool_create(opts)`
 
 Create a connection pool with count of size established connections.
@@ -216,6 +223,12 @@ Return a connection to connection pool.
 
  - `conn` - a connection
 
+### `pool:close()`
+
+Close all connections in pool.
+
+*Returns*: `true`
+
 ## Comments
 
 All calls to connections api will be serialized, so it should to be safe to