Update the 'Best practices for inclusive content' with the newest resources from Diagram Center

[radinamatic]

Best Practices for Authoring MathML in EPUB
http://diagramcenter.org/best-practices-for-authoring-math.html

DIAGRAM Center Best Practice Recommendations for EPUB 3: Now and Looking to the Near Future
http://diagramcenter.org/best-practice-recommendations-for-epub3-now-and-future.html

Diagram Center report on Chemistry
http://diagramcenter.org/diagram-center-report-on-chemistry.html

Best Practices for Authoring Extended Descriptions in EPUB
http://diagramcenter.org/best-practices-for-authoring-extended-descriptions.html