Describe required space after hash as breaking change

[krlmlr]

Follow-up to #4801. Happy to update the website too once we agree on the wording.

[jgm]

The change is already described in the changelog here:

  * Add `space_in_atx_header` extension (#3512).  This is enabled by default
    in pandoc and GitHub markdown but not the other flavors.
    This requires a space between the opening #'s and the header
    text in ATX headers (as CommonMark does but many other implementations
    do not).  This is desirable to avoid falsely capturing things like

        #hashtag

    or

        #5

[jgm]

I don't have objections to adding the proposed text to the changes for the markdown reader, but I wasn't sure if you'd missed the above.

[krlmlr]

When I noticed the new behavior, I skimmed through the release notes, and looked more carefully at the "Changes in behavior" section for the Markdown reader. I don't know what "ATX" means and missed that item. With the new text there's some redundancy, this feature is described from two different angles. I hope that's not too bad.