Add some "known issues" information in the README

[stpaultim]

If I understand correctly, there are several known issues to watch out for while using this module to upgrade a Backdrop site. Including problem with importing files.

I think it would be good to upgrade the README file to alert folks to these possible "gotcha's". Having sat through numerous demos of this module, I'm still a little unsure about which parts I should expect to work, which parts I should skip over, and where to report errors at the database update stage.

I'd be happy to create a PR with change to the Readme if some folks helped me draft the content.

[stpaultim]

After creating a PR with "known issues" in the readme, I thought about creating a WIKI page instead.

I will update my PR to simply link to the Wiki page for known issues. No need to document this stuff in two places and it's probably better to push users to the Wiki.

[stpaultim]

@docwilmot What if we provide an info alert on the START page with a link to the WIKI page (in addition to a link in the README file) of known issues to help people better understand what to watch out for while using this module?

Atten @irinaz

[irinaz]

I vote for list on known issues in one place. Wiki is easier to update, so maybe link to wiki in readme file is better solution than actual list in readme file