readme: improve installation instructions
    
    * Try to redirect people to libtaskotron RPM, in case they just want to
      use it and not develop it.
    * Switch wget to curl, because wget doesn't seem to be often installed
      by default, but curl is.
    * Add python-pip to required packages, it was missing.
    * Merge git installation with the rest of the required packages.
    * Remove resultsdb_api from required packages, it will be installed
      through pip.
    * Make the required packages section easily runnable.
    * Mention in requirements.txt how to install it using package manager
      instead of virtualenv, if people prefer it that way.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D352