Add note about innodb_large_prefix=true requirement (#7212)

[skip ci]

INSTALL

@@ -117,6 +117,11 @@ Note 1: 'password' is the master password for the roundcube user. It is strongly
 recommended you replace this with a more secure password. Please keep in
 mind: You need to specify this password later in 'config/db.inc.php'.
 
+Note 2: When using MySQL < 5.7.7 or MariaDB < 10.2.2 it is required to configure
+the database engine with:
+    innodb_large_prefix=true
+    innodb_file_format=Barracuda
+
 
 * SQLite
 --------

UPGRADING

@@ -23,6 +23,9 @@ removed again.
 WARNING: Make sure files have proper owner/group for your setup. If you use
          tar to extract the package, `--no-same-owner` option might be helpful.
 WARNING: See Post-Upgrade Activities section below.
+WARNING: If you use MySQL < 5.7.7 or MariaDB < 10.2.2 make sure to configure it with:
+            innodb_large_prefix=true
+            innodb_file_format=Barracuda
 
 
 Updating manually