README.md in hydra-derivatives-3.5.0 vs README.md in hydra-derivatives-3.6.0

@@ -193,20 +193,25 @@
 MAGICK_TEMPDIR
 
 ```
 YMMV as to where setting them will take effect in your app; the application's web server's vhost directives are a location known to work with an Apache web server set up.
 
+ImageMagick by default disables reading and writing of PDFs due to a [vulnerability in ghostscript](https://www.kb.cert.org/vuls/id/332928/). Make sure to install a fixed version of ghostscript and modify ImageMagick's security policy to allow reading and writing PDFs:
+```
+sudo sed -i 's/policy domain="coder" rights="none" pattern="PDF"/policy domain="coder" rights="read|write" pattern="PDF"/' /etc/ImageMagick-6/policy.xml
+```
+
 ## JPEG2k Directives
 
 Unlike the other processors, the `Jpeg2kImage` processor does not generally accept arguments that directly (or nearly so) translate to the arguments you would give to the corresponding command line utility.
 
 Instead, each directive may contain these arguments:
 
   * `:output_path` (String) : The name for the new file
   * `:to_srgb` (Boolean) : If `true` and the image is a color image it will map the source image color profile to sRGB. Default: `true`
   * `:resize` (String) : Geometry; the same syntax as the `Hydra::Derivatives::Image` processor
   * `:recipe` :
-    - If a Symbol the recipe will be read from the `Hydra::Derivatives.kdu_compress_recipes` hash. You can override this, or a couple of samples are supplied. The symbol in the config file should be the name in the model + `_{quality}`, e.g. `recipe: :default` will look `:default_color` or `:default_grey` in the hash.
+    - If a Symbol the recipe will be read from the `Hydra::Derivatives.kdu_compress_recipes` hash. You can override this, or a couple of samples are supplied. The symbol in the config file should be the name in the model + `_{quality}`, e.g. `recipe: :default` will look `:default_color` or `:default_gray` in the hash.
     - If a String the recipe in the string will be used. You may include anything the command line utility will accept except `-i` or `-o`. See `$ kdu_compress -usage` in your shell.
     - If no `:recipe` is provided the processor will examine the image and make a best guess, but you can set a few basic options (the remainder of this list). Note that these are ignored if you provided a recipe via either of the first two methods described.
   * `:levels` (Integer) : The number of decomposition levels. The default is the number of times the long dimension can be divided by two, down to 96, e.g. a 7200 pixel image would have 6 levels (3600, 1800, 900, 450, 225, 112)
   * `:layers` (Integer) : The number of quality layers. Default: 8
   * `:compression` (Integer) : The left number of the compression ratio `n:1`, e.g. 12 will apply 12:1 compression. Default: 10.