Update docs

README.md

@@ -37,6 +37,19 @@ Or install it yourself as:
 
     $ gem install filestack
 
+## Dependencies
+
+We use the gem `mimemagic` which requires a copy of the Freedesktop.org shared-mime-info database.
+
+macOS users can install the database via Homebrew with `brew install shared-mime-info`.
+
+If you are unable to use a package manager, you can obtain a copy of the needed file by extracting it from the Debian package. This process will also work on a Windows machine.
+
+1. Download the package from https://packages.debian.org/sid/amd64/shared-mime-info/download
+2. Ensure the command line version of 7-Zip is installed
+3. `7z x -so shared-mime-info_2.0-1_amd64.deb data.tar | 7z e -sidata.tar "./usr/share/mime/packages/freedesktop.org.xml"`
+4. Place the file `freedesktop.org.xml` in an appropriate location, and then set the environment variable `FREEDESKTOP_MIME_TYPES_PATH` to that path. Once that has been done the gem should install successfully. Please note that the gem will depend upon the file remaining in that location at run time.
+
 ## Usage
 
 ### Import