docs: update README.md features

[eonu]

Update README.md features

• Add speed section to introduction.
• Mention multi-processed training for HMMClassifier.
• Mention fallback for dtaidistance C libraries.

Checklist

• I have added new tests (if necessary).
• I have ensured that tests and coverage are passing on CI.
• I have updated any relevant documentation (if necessary).
• I have used a descriptive pull request title.

[coveralls]

Pull Request Test Coverage Report for Build d9a5d899-a317-47ef-a3c9-15a84b4036f3

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.569%

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Totals
Change from base Build ac0fbb9f-f597-4ef1-9ded-0c27bf9c5712:	0.0%
Covered Lines:	1170
Relevant Lines:	1417

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💛 - Coveralls