Update TTLowpassLinkwitzRiley2.cpp

original URL with the article seems outdated, replacing it with a working URL
Update TTLowpassLinkwitzRiley4.cpp

original URL with the article seems outdated, replacing it with a working URL
Merge branch 'dev'
Create README.md
Adding waffle badge
Merge branch 'dev'
Merge branch 'dev' into merge/api_and_api-cleanup