07/04/2021: Add some form of build documentation to either readme or contributing

[bjthorpe]

As a new user, I noticed there is a severe lack of instructions on how to check or build the glossario. The readme makes a passing mention to Jeckyl (google was my friend in that case) and contributing just has the line "In case you wish to build up the website locally to double-check the final look, you can use make serve" which if you use it without first instaling Jeckyl gives a bunch of confusing errors. So should we add installation instructions for Jeckyl? or at least a link to them as there is no need to get too detailed here. Also, I would perhaps suggest we expand the section in contributing to include make check.

[bjthorpe]

My idea for a possible fix #301

[baileythegreen]

Just left some feedback on the PR.