Added Euler Number and reserved variables to user docs

[m-messer]

No description provided.

[peterbjohnson]

Thanks for adding this info. A couple of suggestions:

• add the 'hack' that input symbols can be used to allow reserved symbols to be used for different purposes
• reorganise the content so that all info on reserved symbols is together. E.g. overriding Greek symbols; using input symbols; and elementary functions. Maybe there's not a perfect solution, or maybe there's not even a better solution - but it's worth a look.

Also regarding branches, Karl previously used Trello tickets, and labelled branches with the Trello ticket number. You can use that system, or a different system (e.g. GH issue numbers, or your own system). Whatever you use, can you then formalise that somewhere, e.g. in the README or the general docs, so that we document good practice.

[m-messer]

Pulled comment to GH issue