Code Review #6326
docs are not re-build when grf or version changes
The readme may contain next to the version-specific title, the NewGRF version. This currently is not updated upon re-build.
r56 :61df8140c1f8 fixes this in a somewhat ugly way. We maybe rather want to store the information inserted into the readme somewhere and compare the results to the current run. And only re-build it when required.