update readme to separately install float_raster

README.md

@@ -19,4 +19,5 @@ Requirements:
 
 Install with pip, via git:
 
+>pip install --upgrade git+https://mpxd.net/gogs/jan/float_raster.git@release
 >pip install --upgrade git+https://mpxd.net/gogs/jan/gridlock.git@release