README.md in snapshot-0.2.4 vs README.md in snapshot-0.3.0

@@ -165,16 +165,18 @@
 ### Scheme
 To not be asked which scheme to use, just set it like this:
 ```ruby
 scheme "Name"
 ```
+You can also use the environment variable ```SNAPSHOT_SCHEME```.
 
 ### Screenshots output path
 All generated screenshots will be stored in the given path.
 ```ruby
 screenshots_path './screenshots'
 ```
+You can also use the environment variable ```SNAPSHOT_SCREENSHOTS_PATH```.
 
 ### Project Path
 By default, ```snapshot``` will look for your project in the current directory. If it is located somewhere else, pass your custom path:
 ```ruby
 project_path "./my_project/Project.xcworkspace"
@@ -196,9 +198,40 @@
 ### Custom Build Command
 If for some reason, the default build command does not work for your project, you can pass your own build script. The script will be executed **once** before the tests are being run.
 
 **Make sure** you are setting the output path to ```/tmp/snapshot```.
 
+```ruby
+build_command "xcodebuild DSTROOT='/tmp/snapshot' OBJROOT='/tmp/snapshot' SYMROOT='/tmp/snapshot' ... "
+```
+
+### Custom callbacks to prepare your app
+Run your own script when ```snapshot``` switches the simulator type or the language. 
+This can be used to 
+- Logout the user
+- Reset all user defaults
+- Pre-fill the database
+
+To run a shell script, just use ```system('./script.sh')```.
+```ruby
+setup_for_device_change do |device| 
+  puts "Preparing device: #{device}"
+end
+
+setup_for_language_change do |lang, device|
+  puts "Running #{lang} on #{device}"
+  system("./popuplateDatabase.sh")
+end
+
+teardown_language do |lang, device|
+  puts "Finished with #{lang} on #{device}"
+end
+
+teardown_device do |device|
+  puts "Cleaning device #{device}"
+  system("./cleanup.sh")
+end
+```
 
 # Tips
 ## Available language codes
 ```ruby
 ["da-DK", "de-DE", "el-GR", "en-AU", "en-CA", "en-GB", "en-US", "es-ES", "es-MX", "fi-FI", "fr-CA", "fr-FR", "id-ID", "it-IT", "ja-JP", "ko-KR", "ms-MY", "nl-NL", "no-NO", "pt-BR", "pt-PT", "ru-RU", "sv-SE", "th-TH", "tr-TR", "vi-VI", "cmn-Hans", "zh_CN", "cmn-Hant"]