chore: update README with deprecation notes

[michael-siek]

Partially Closes issue:

dequelabs/axe-core-npm#4

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[jeeyyy]

@michael-siek do not forget to update the about/ topics and description on the home page.

See https://github.com/dequelabs/axe-reporter-earl for reference.

[stephenmathieson]

Should we archive these repos after adding this notice? Or are we going to keep them alive for awhile so folks have time to update?