docs(readme): Add message in README steering people to parse5 if they need HTML5 compliance

[vassudanagunta]

I think this is important to steer people to htmlparser2 or parse5 based on this key difference, especially since this difference is fundamental and permanent. It's important to do this up front to save people the time and effort of finding out the hard way.

Fixes #1075

Happy to reword or make any other changes you want, or drop this and leave it all to you :)

[coveralls]

Pull Request Test Coverage Report for Build 2212190919

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.5%) to 99.426%

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Files with Coverage Reduction	New Missed Lines	%
src/fixtures/test-helper.ts	3	97.45%
src/Tokenizer.ts	7	99.14%

Totals
Change from base Build 2082618917:	0.5%
Covered Lines:	1907
Relevant Lines:	1918

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💛 - Coveralls

[vassudanagunta]

@fb55 This may also "fix" #1076 ?

The lint check failures pre-existing, unrelated to this PR.