Update collations and make utf8mb4 default (go-sql-driver#877)

Author: methane

README.md

@@ -171,13 +171,18 @@ Unless you need the fallback behavior, please use `collation` instead.
 ```
 Type:           string
 Valid Values:   <name>
-Default:        utf8_general_ci
+Default:        utf8mb4_general_ci
 ```
 
 Sets the collation used for client-server interaction on connection. In contrast to `charset`, `collation` does not issue additional queries. If the specified collation is unavailable on the target server, the connection will fail.
 
 A list of valid charsets for a server is retrievable with `SHOW COLLATION`.
 
+The default collation (`utf8mb4_general_ci`) is supported from MySQL 5.5.  You should use an older collation (e.g. `utf8_general_ci`) for older MySQL.
+
+Collations for charset "ucs2", "utf16", "utf16le", and "utf32" can not be used ([ref](https://dev.mysql.com/doc/refman/5.7/en/charset-connection.html#charset-connection-impermissible-client-charset)).
+
+
 ##### `clientFoundRows`
 
 ```