add _stmt_cache_size to DSN parameters table in README

Author: mattn

README.md

@@ -125,6 +125,7 @@ Boolean values can be one of:
 | Transaction Lock | `_txlock` | <ul><li>immediate</li><li>deferred</li><li>exclusive</li></ul> | Specify locking behavior for transactions. |
 | Writable Schema | `_writable_schema` | `Boolean` | When this pragma is on, the SQLITE_MASTER tables in which database can be changed using ordinary UPDATE, INSERT, and DELETE statements. Warning: misuse of this pragma can easily result in a corrupt database file. |
 | Cache Size | `_cache_size` | `int` | Maximum cache size; default is 2000K (2M). See [PRAGMA cache_size](https://sqlite.org/pragma.html#pragma_cache_size) |
+| Statement Cache Size | `_stmt_cache_size` | `int` | Maximum number of prepared statements cached per connection; default is 0 (disabled). Note that `sql.DB` is a connection pool, so each connection maintains its own independent cache. |
 
 
 ## DSN Examples