Add simple mention of Docker images to installation doc.
Add simple mention of Docker images to installation doc.

rc/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy,BubuLK/sfepy,vlukes/sfepy,sfepy/sfepy,sfepy/sfepy,rc/sfepy