Split up and rewrite documentation

The README file has been split up into separate files for installation
and development. A new documentation for the upgrade process has been
added.

The following changes have been made regarding content:
* Removed outdated Vagrant section
* Replaced unnecessary session persistance section
* Use simple HTTP proxy module instead of AJP for Apache httpd
* Nginx as alternative for Apache httpd
* Use Websocket proxy which makes HTTPS configuration on the Java side
  obsolete
* Additional configuration steps for server software
* Added requirements section
* Added upgrading instructions