Update readme and bump version to 0.4.0

Author: mathieuprog

README.md

@@ -8,7 +8,23 @@ module can, by default, only operate on datetimes in the UTC time zone. Alternat
 third-party libraries, such as `tz`, to bring in time zone support and deal with datetimes in other time zones than UTC.
 
 The `tz` library relies on the [time zone database](https://data.iana.org/time-zones/tzdb/) maintained by
-[IANA](https://www.iana.org). As of version 0.3.0, `tz` uses version _tzdata2019c_ of the IANA time zone database.
+[IANA](https://www.iana.org). As of version 0.4.0, `tz` uses version _tzdata2019c_ of the IANA time zone database.
+
+## Features
+
+### Battle-tested
+
+The `tz` library is tested against nearly 10 million past dates, which includes most of all possible imaginable
+edge cases.
+
+### Pre-compiled time zone data into maps
+
+Time zone periods are deducted from the [IANA time zone data](https://data.iana.org/time-zones/tzdb/). A period is a
+period of time where a certain offset is observed. Example: in Belgium, from 31 March 2019 until 27 October 2019, clock
+went forward by 1 hour; this means that during this period, Belgium observed a total offset of 2 hours from UTC time.
+
+The time zone periods are computed and made available in Elixir maps during compilation time, to be consumed by the
+[DateTime](https://hexdocs.pm/elixir/DateTime.html#module-time-zone-database) module.
 
 ## Usage
 
@@ -37,12 +53,11 @@ Add `tz` for Elixir as a dependency in your `mix.exs` file:
 ```elixir
 def deps do
   [
-    {:tz, "~> 0.3.0"}
+    {:tz, "~> 0.4.0"}
   ]
 end
 ```
 
 ## HexDocs
 
 HexDocs documentation can be found at [https://hexdocs.pm/tz](https://hexdocs.pm/tz).
-

mix.exs

@@ -1,7 +1,7 @@
 defmodule Tz.MixProject do
   use Mix.Project
 
-  @version "0.3.0"
+  @version "0.4.0"
 
   def project do
     [