diff --git a/.travis.yml b/.travis.yml
index a9b54be..6ede6be 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,3 +13,5 @@ script:
   - cargo check --target thumbv7em-none-eabihf --no-default-features --features "microfft-complex"
   - cargo check --target thumbv7em-none-eabihf --no-default-features --features "microfft-real"
 
+  # examples
+  - cargo run --release --example mp3-samples
diff --git a/CHANGELOG.md b/CHANGELOG.md
index f17d192..9f3d5d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,14 @@
 # Changelog
 
+## v0.4.0
+- MSRV is now Rust 1.51 (sorry but that's the only way I can make it `no_std`-compatible)
+- This crate is now really `no_std`
+- you can choose between three FFT implementations at compile time via Cargo features.
+  The new default feature is `rustfft-complex` (which is still `std`) but there are also 
+  the two new `no_std`-compatible targets `microfft-complex` (more accurate, like rustfft)
+  and `microfft-real` (faster, less accurate)
+- several small improvements and fixes, see: https://github.com/phip1611/spectrum-analyzer/milestone/3?closed=1
+
 ## v0.3.0
 - `FrequencySpectrum::min()` and `FrequencySpectrum::max()`
    now return tuples/pairs (issue #6)
diff --git a/Cargo.toml b/Cargo.toml
index 656eb91..959f661 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,7 +4,7 @@ description = """
 A simple and fast `no_std` library to get the frequency spectrum of a digital signal (e.g. audio) using FFT.
 It follows the KISS principle and consists of simple building blocks/optional features.
 """
-version = "0.3.0"
+version = "0.4.0"
 authors = ["Philipp Schuster <phip1611@gmail.com>"]
 edition = "2018"
 keywords = ["fft", "spectrum", "frequencies", "audio", "dsp"]
diff --git a/EDUCATIONAL.md b/EDUCATIONAL.md
new file mode 100644
index 0000000..e247a73
--- /dev/null
+++ b/EDUCATIONAL.md
@@ -0,0 +1,23 @@
+## How to use FFT to get a frequency spectrum?
+
+This library is full of additional and useful links and comments about how an FFT result 
+can be used to get a frequency spectrum. In this document I want to give a short introduction 
+where inside the code you can find specific things.
+
+**TL;DR:** Although this crate has 1400 lines of code, **the part which gets the frequency and
+their values from the FFT is small and simple**. Most of the code is related to my convenient 
+abstraction over the FFT result including several getters, transform/scaling functions, and
+tests.
+
+**I don't explain how FFT works but how you use the result!**
+If you want to understand that too:
+
+- check out all links provided [at the end of README.md](/README.md)
+- look into `lib.rs` (**probalby gives you 90 percent of the things you want to know**) 
+  and the comments over the FFT abstraction in `src/fft/mod.rs` and 
+  `src/fft/rustfft-complex/mod.rs`.
+  
+
+This is everything important you need. Everything inside 
+ `spectrum.rs` and the other files is just convenient stuff + tests for when you 
+want to use this crate in your program.
diff --git a/README.md b/README.md
index cb5a087..8653f0c 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,9 @@ via Cargo features.
 **The MSRV (minimum supported Rust version) is 1.51 Stable because this crate needs the 
 "resolver" feature of Cargo to cope with build problems occurring in `microfft`-crate.**
 
+## I want to understand how FFT can be used to get a spectrum
+Please see file [/EDUCATIONAL.md](/EDUCATIONAL.md).
+
 ## How to use (including `no_std`-environments)
 Most tips and comments are located inside the code, so please check out the repository on
 Github! Anyway, the most basic usage looks like this:
diff --git a/check-build.sh b/check-build.sh
index 67fd36a..ecb2e1e 100644
--- a/check-build.sh
+++ b/check-build.sh
@@ -6,3 +6,7 @@ cargo test --all-targets
 rustup target add thumbv7em-none-eabihf
 cargo check --target thumbv7em-none-eabihf --no-default-features --features "microfft-complex"
 cargo check --target thumbv7em-none-eabihf --no-default-features --features "microfft-real"
+
+
+# run examples
+cargo run --release --example mp3-samples