Update readme

README.md

@@ -1,11 +1,20 @@
 # pdfsearch #
 
-**pdfsearch** finds text strings inside a pdf.
+**pdfsearch** finds text strings inside a pdf. It relies on `pdftotext` from the [Poppler](https://poppler.freedesktop.org/) library.
 
 ## Installation ##
 
+You must install Poppler, and add it to your search path.
+
+**Windows**: http://blog.alivate.com.au/poppler-windows
+
+**Linux**: `sudo apt install poppler-utils`
+
+**Mac**: `brew install poppler`
+
+Then
+
 ```
-pip install PyPDF2
 pip install tqdm
 ```