I don't find a section "Installing from a Git clone" in the readme. I assume you refer to the readme for Geany.
Correct, it was added in geany/geany#1087 for Geany's README, but no equivalent for Geany-Plugins'.
FWIW, if you want to use an actual release version, it's simpler to use [a release tarball](https://plugins.geany.org/downloads.html) which contains all the generated files that aren't checked into Git, and so also doesn't require additional Autotoools programs to generate those files. This is the type of build the README currently assumes.
Related: #379