Update README.md

Author: GauteR

README.md

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 A collaboration on the MathML guidelines for marking up accessible math
 
+Available at <https://nlbdev.github.io/mathml-guidelines/>
+
 ## Nordic MathML guidelines: Use, validation and feedback
 
 We now have clear, detailed, and shared guidelines for MathML markup. To keep using a common set of guidelines going forward, we should establish shared procedures for how we validate, give feedback to the Indian suppliers and report issues.