Document intentional design choices where different from webapp/mobile

[neiljp]

This issue has some overlaps with the idea of having tutorials or extra documentation along these lines, but my thought for this issue would be a small section in the README indicating eg. how ZT is keyboard driven and only incidentally mouse-accessible (for now), and other related design choices/constraints.

Most aspects of ZT have tried to remain similar to the webapp, but it would similarly be useful to document specific cases where we are not compatible, or have extensions which are different/interesting (eg. z key)

This could also be an area where we list known limitations, whether inherent to the platform (ie. images) or where we don't yet support certain features or ranges of features, and potentially the priority we ascribe to adding them right now. This potentially has overlap with just tracking with issues, but would probably be broader unsupported areas, eg. admin or options.

[neiljp]

I've updated this issue title to narrow the scope slightly.

Listing known limitations probably belong with #731, though longer-term aspects are now part of https://github.com/zulip/zulip-terminal#about

When we add the details of intentional differences, which could perhaps go into a separate file in docs/, we can also consider relating these to a FAQ entry named something like "Migrating from the webapp (or other Zulip interfaces)" - or this could even be in the README.

Note that as briefly discussed in #26, this covers intentional differences and those imposed through platform requirements.