Added missing <tbody> tags after <table> in mjml-hero, mjml-button and mjml-social components

[Pharmasolin]

I noticed that mjml-hero, mjml-button and mjml-social components didn't have <tbody> and </tbody>, this might add confusion for developers who might want to use mj-html-attributes component and css child combinator >.

[Pharmasolin]

I think I messed PR a bit, I found that README.md has Contribute link which points to nowhere, but there is CONTRIBUTING.md file in repository and I think it should point there?

Please let me know if this url change in readme is not needed, I will resubmit PR then. (I forget to create new branch in my fork for this PR)

[iRyusa]

This was a reference to the old 3.X branch here https://github.com/mjmlio/mjml/tree/3.3.x#contribute I think you can remove it too

[Pharmasolin]

@iRyusa should I revert change in last commit?

[iRyusa]

You can juste remove the link and mention i'll squash it

[Pharmasolin]

@iRyusa removed contribution reference in readme.

[kmcb777]

thanks for this PR 👍
i think this should wait for v5 to be merged because it could break existing templates that use css selectors

[Pharmasolin]

@kmcb777 100% would love to see it in v5 (I will need to update some of my templates too).

[iRyusa]

Even if it breaks a bit the DOM I think we should merge this in 4.10 as it's way better for accessibility cc @Pharmasolin can you resolve the conflict so we get this merged ?

[Pharmasolin]

@iRyusa resolved the conflict, but please check if everything is ok before merging this branch to master. For example it has change to readme.md as well.

[iRyusa]

Looks good to me this section no longer exist in the readme anyway :D