add repo for markdown and html tables

[junguler]

hi, i love what you've done with these repos, i would appreciate a write up about tables in markdown and the html tags that are available in github's markdown

normally i wouldn't think there was a possibility of custom tables other than using the pipes | but this repo has an example of html tables in markdown but unfortunately it's quite advanced and i wasn't really able to reverse engineer it for my use cases

https://github.com/microsoft/AI-For-Beginners/blob/main/README.md#content

basically i want to know if it's possible to have a header that takes the whole width of the table and not have separate cells that are empty