README: document DSN system var quoting rules (go-sql-driver#405)

Improve documentation of quoting rules for system var values in DSN.
go-sql-driver#405

Author: dolmen

README.md

@@ -315,13 +315,21 @@ I/O write timeout. The value must be a decimal number with an unit suffix ( *"ms
 
 ##### System Variables
 
-All other parameters are interpreted as system variables:
-  * `autocommit`: `"SET autocommit=<value>"`
-  * [`time_zone`](https://dev.mysql.com/doc/refman/5.5/en/time-zone-support.html): `"SET time_zone=<value>"`
-  * [`tx_isolation`](https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_tx_isolation): `"SET tx_isolation=<value>"`
-  * `param`: `"SET <param>=<value>"`
+Any other parameters are interpreted as system variables:
+  * `<boolean_var>=<value>`: `SET <boolean_var>=<value>`
+  * `<enum_var>=<value>`: `SET <enum_var>=<value>`
+  * `<string_var>=%27<value>%27`: `SET <string_var>='<value>'`
+
+Rules:
+* The values for string variables must be quoted with '
+* The values must also be [url.QueryEscape](http://golang.org/pkg/net/url/#QueryEscape)'ed!
+ (which implies values of string variables must be wrapped with `%27`)
+
+Examples:
+  * `autocommit=1`: `SET autocommit=1`
+  * [`time_zone=%27Europe%2FParis%27`](https://dev.mysql.com/doc/refman/5.5/en/time-zone-support.html): `SET time_zone='Europe/Paris'`
+  * [`tx_isolation=%27REPEATABLE-READ%27`](https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_tx_isolation): `SET tx_isolation='REPEATABLE-READ'`
 
-*The values must be [url.QueryEscape](http://golang.org/pkg/net/url/#QueryEscape)'ed!*
 
 #### Examples
 ```