Document the support of PostgreSQL, MariaDB, MySQL in README

Author: nene

README.md

@@ -89,6 +89,9 @@ The plugin provides the following parsers:
 
 - `sqlite`
 - `bigquery`
+- `postgresql` (**experimental! expect crashes**)
+- `mysql` (**experimental! expect crashes**)
+- `mariadb` (**experimental! expect crashes**)
 
 ## Configuration
 
@@ -107,6 +110,18 @@ Currently this plugin supports two SQL dialects:
 - **SQLite** - full support.
 - **BigQuery** - full support.
 
+It also has experimental support for the following dialects:
+
+- PostgreSQL
+- MySQL
+- MariaDB
+
+The main limitation is that [the parser][sql-parser-cst] does not support full syntax of
+these dialects. One should expect the parser to crash for syntax that's more specific to
+these dialects. But as long as the parsing succeeds, the formatting should also succeed.
+Mainly one can expect the formatting of SELECT statements to work. But beyond that you should
+keep your expectations low. You have been warned.
+
 The specifics of the [SQL formatting style][STYLE_GUIDE] are still very much subject to change.
 Though the general principles should be mostly in place by now.