Fix readme (#186)

Author: swissspidy

.readme-partials/USING.md

@@ -201,6 +201,7 @@ The following environment variables can be set to override the default database
   - `WP_CLI_TEST_DBNAME` is the database that the tests run under (defaults to "wp_cli_test").
   - `WP_CLI_TEST_DBUSER` is the user that the tests run under (defaults to "wp_cli_test").
   - `WP_CLI_TEST_DBPASS` is the password to use for the above user (defaults to "password1").
+  - `WP_CLI_TEST_DBTYPE` is the database engine type to use, i.e. "sqlite" for running tests on SQLite instead of MySQL (defaults to "mysql").
 
 Environment variables can be set for the whole session via the following syntax: `export WP_CLI_TEST_DBNAME=custom_db`.