[README] Update link of user community site

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| Refs tickets  | none
| License       | MIT
| Doc PR        | none

http://community.zikula.org/ not work, i change it to http://zikula.org/index.php/forum/