[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/
please remove the `index.php/`

zikula.org/forum should work