Update free-programming-books.md

Added category for embedded programming with three starting resources

free-programming-books.md

@@ -39,6 +39,7 @@ Original Source: [List of freely available programming books](http://web.archive
 * [Elasticsearch](#elasticsearch)
 * [Elixir](#elixir)
 * [Emacs](#emacs)
+* [Embedded Programming](#embedded-programming)
 * [Erlang](#erlang)
 * [F#](#f-sharp)
 * [Firefox OS](#firefox-os)
@@ -909,6 +910,12 @@ Original Source: [List of freely available programming books](http://web.archive
 * [GNU Emacs Manual, 17th Edition, v. 24.2](http://shop.fsf.org/product/Emacs_Manual_24/)
 
 
+### Embedded Programming
+* [Control and Embedded Systems](http://www.learn-c.com/) (HTML)
+* [Discovering the STM32 Microcontroller](http://www.cs.indiana.edu/~geobrown/book.pdf) (PDF)
+* [Introduction to Microcontrollers](http://www.embeddedrelated.com/showarticle/453.php) (HTML)
+
+
 ### Erlang
 * [Concurrent Programming in ERLANG](http://www.erlang.org/download/erlang-book-part1.pdf)
 * [Erlang Handbook](https://github.com/esl/erlang-handbook/raw/master/output/ErlangHandbook.pdf) (PDF)