Update README.md

[ticaki]

add missing #

[mcm1957]

@AlCalzone Note While zhe example must gsve 3 # for README.md it might be correct to havebonly 2 for CHANGELOG.md

Maybe this must be described in more Derail to avoid breaking working with CHANGELOG.nd while correnting README.md.

[AlCalzone]

This isn't correct. The changelog entries in README have 3 #, while the ones in CHANGELOG only have 2 #.

The documentation is also very clear about this:

If anything, we could add a sentence under usage to mention this again:

The following assumes that you keep the changelog in `CHANGELOG.md`. If you keep it in `README.md`, prepend another `#`.

or so...