update installation and execution instructions

[rekado]

the README should mention:

• how to run the driver script
• to set PIGX_RNASEQ_UNINSTALLED for running the pipeline from a source checkout
• explain the GNU build system (./bootstrap, ./configure, make install)
• how to install with Guix
• how to build a docker image with Guix (make docker)

Maybe unify instructions with pigx_bsseq.