more README

README.md

@@ -4,4 +4,18 @@ A set of scripts for doing things with PDF
 
 ## compression
 
-compresspdf.sh is a script made by members of [Open Source Publishing](http://osp.kitchen) that squashes large PDFs into very small ones using ```gs```, an open-source software suite for interpreting PostScript and PDF.
+compresspdf.sh is a script made by members of [Open Source Publishing](http://osp.kitchen) that squashes large PDFs into very small ones using ```gs``` (ghostscript), an open-source software suite for interpreting PostScript and PDF.
+
+First download and install ghostscript from here: https://www.ghostscript.com/
+
+To run the script, do:
+
+```./compresspdf.sh <file.pdf> <resolution in DPI>```
+
+For example:
+
+```./compresspdf.sh input.pdf 300```
+
+The output will be something like:
+
+```input-resampled.pdf```