Update readme for app form exercises 6 chapter 13

Author: Bernard Pietraga

chapter_13/cracow_weather/README.md

@@ -1,8 +1,61 @@
 # CracowWeather
 
-**TODO: Add description**
+This application is solution for exercise 6 of chapter 13 written in
+`Programming Elixir` by `Dave Thomas`. It allows user to quickly fetch weather
+data from Cracow, Poland and print it to terminal.
+
+It is also executable Erlang Escript.
 
 ## Installation
+You need to have Elixir version `1.4` installed localy.
+
+First download dependencies.
+```
+mix deps.get
+```
+
+Than you can type run to compile
+```
+mix run
+```
+
+## Usage via IEx
+Go to root path of this app.
+
+Compile and load CracowWeather to Iex.
+```
+iex -S mix
+```
+
+Print latest weather data to your terminal.
+```
+CracowWeather.CLI.main
+```
+
+## Usage as a shell script via escript
+Make sure you have Erlang installed
+
+Build fresh version of script
+```
+mix escript.build
+```
+
+Run script via shell.
+```
+./cracow_weather
+```
+
+## Tests
+To run tests simply type
+```
+mix test
+```
+
+## Documentation
+To generate documentation via ex_doc please type
+```
+mix docs
+```
 
 If [available in Hex](https://hex.pm/docs/publish), the package can be installed
 by adding `cracow_weather` to your list of dependencies in `mix.exs`: