docs: update README.md

[MichaelCurrin]

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).

[MichaelCurrin]

I noticed the Travis link goes here

https://travis-ci.org/github/nodemcu/nodemcu-firmware

Which is deprecated

and recommends going here. So I used this

https://travis-ci.com/github/nodemcu/nodemcu-firmware

[marcelstoer]

Some of those changes are subjective. As for Travis, we should actually remove .travis.yml (as we don't need it anymore) and instead add the shield for our GitHub workflow.

And this really should land on dev instead of release as the template requires.

[MichaelCurrin]

Thanks. I've removed the more subject changes.

Now it's mostly about making H1s into H2s (accessibility scores such in SEO engines will penalize you will having more than one H1 on a page).

I've replaced with a GH Actions badge.

Sorry I misunderstood how the dev/release bit worked.

[HHHartmann]

do we want to also take this PR? Why is ist against release and not dev?

[marcelstoer]

I'll clean this up and rebase on dev.