This page seems to be the canonical introduction to writing a library: https://www.arduino.cc/en/Hacking/LibraryTutorial

However, that page does not mention the src/ directory and library.properties at all, so it probably has not been updated in a long while. Since we want to encourage people to include a library.properties file with their library (it should not be an optional extra, every library should ideally have it), that page should be updated with the new library format in mind (with a library.properties example, and probably using the src/ subfolder, but perhaps mention the alternative without src/ as well). It should probably also mention version numbers (not) supporting the new recursive format, just in case people also need to support old 1.0.x versions (IIRC the latest 1.0.x versions also support the new format).