Update Readme based on debian/ubuntu command (#95)

Author: hmtanbir

README.md

@@ -29,10 +29,18 @@ how to resize and process images.
 
 1. Install ImageMagick and/or libvips:
 
-  ```sh
+In a Mac terminal:
+  
+```sh
   $ brew install imagemagick vips
   ```
 
+ In a debian/ubuntu terminal:
+
+```sh
+  $ sudo apt install imagemagick libvips
+  ```
+
 2. Add the gem to your Gemfile:
 
   ```rb
@@ -181,10 +189,17 @@ pipeline
 
 Our test suite requires both `imagemagick` and `libvips` libraries to be installed.
 
+In a Mac terminal:
+
 ```
 $ brew install imagemagick vips
 ```
 
+In a debian/ubuntu terminal:
+```shell
+sudo apt install imagemagick libvips
+```
+
 Afterwards you can run tests with
 
 ```