Currently, .md files intended to be used as READMEs in the amphtml extensions directory are named after the extension (e.g. amp-font.md) rather than README.md.

An issue (ampproject/amphtml#1304) was created in amphtml to rename the extension markdown files to README.md. As mentioned in the issue, for that change to be made, the documentation build scripts need to be modified to accept the renaming.