Improve initial setup instructions

[waldyrious]

I've just helped an admin of a Telegram set up the bot for that chat, and we've succeeded, but bumped into a couple issues interpreting the instructions. I suppose they're possibly a bit too techy; in particular:

• the regex-style "or" syntax: (normallinks|wikibaselinks)
• the dollar-prefixed variable name: $URL

Other than that it's pretty good. Maybe a concrete example next to the general pattern could help drive the point home :)

It's also not super clear that the commands are supposed to be executed in the desired chat room, not in the conversation with the bot.

Finally, it might be worth providing an explanation of what "normallinks" and "wikibaselinks" mean.