dokuwiki: some DokuWiki plugins may require additional PHP packages

[mhenry07]

It would be helpful to add a note to dokuwiki/README.md that some DokuWiki plugins may require installing additional PHP packages in order to work properly.

Specifically, when attempting to use plugin:oauth, I was getting the following errors in my /var/log/nginx/error.log:

[error] 56#56: *148 FastCGI sent in stderr: "PHP message: PHP Fatal error: Call to undefined function OAuth\OAuth2\Service\json_decode() in /var/www/html/lib/plugins/oauth/phpoauthlib/src/OAuth/OAuth2/Service/Generic.php on line 80" while reading response header from upstream.

In this case, the solution was install the Alpine package php5-json, either by updating the apk add command in Dockerfile or by adding a custom command to docker-compose.yml, e.g.
command: apk add --no-cache php5-json && php-fpm5 && nginx -g 'daemon off;'.

See also: cosmocode/dokuwiki-plugin-oauth#60