Fix Proposals and Troubles with Source

[Jaden Rodriguez]

I have a couple of fixes I think would be easy to implement but am having trouble working with source (I think it is compatibility with my OS but not sure).

Namely, there are a couple of updates to the core docs that I think would make them more concise and allow the focus to lie more on Gensim than implementation of small details (small impact).

I also noticed there are issues with the downloader/API which have a cryptic error message with a relatively easy fix once you identify the problem. An error message says "can't read cache, connect to internet", but is resolved by updating SSL certificate via pip. An automatic certificate update or even just a more informative error response would help resolve this problem I've seen others run into (larger impact). 

See https://stackoverflow.com/questions/64425652/valueerror-when-downloading-gensim-data-set.

Would love to contribute myself but some guidance would be nice.

[Gordon Mohr]

Without more details, unsure what specific source errors you're having. A general guide to contributing, via Pull Requests (PRs) with proposed code changes, is at <https://github.com/RaRe-Technologies/gensim/blob/develop/CONTRIBUTING.md>.

But, it's the same general flow as with any public Github-hosted project - so if you're hitting OS/configuration specific issues, any resources/answers addressing those issues, with regard to anyone trying to locally-edit/share-back a codebase, might help. (And, even if the full process seems involved & ornery at 1st, once you get the hang of fork/clone/local-install/tweak/commit/push-back, it's a pattern you'll be able to reuse to adapt, & contribute-back-to, many open-source projects.)

With regard to the certificate issue, I'd not want to automatically install any packages for a user, as that's a step I think should be under conscious, explicit decision. But, code which catches such errors, & shows a message pointing users in the right direction – even to the level of suggesting the exact install command that is likely to resolve the problem – would seem to be a helpful & welcome addition. 

- Gordon