[GEOS-6379] Unable to add new default gridsets
Updated GDAL plugin README.txt
Merge pull request #688 from n-lagomarsini/GEOS-6379_2_5

[GEOS-6379] Unable to add new default gridsets (Backport on 2.5.x)
Merge pull request #689 from n-lagomarsini/update_readme

Updated GDAL plugin README.txt