Add FreeRTOS book (#5469)

* Add FreeRTOS book

Resolves part of EbookFoundation/free-programming-books/issues/5466

* Move FreeRTOS book to Embedded Systems

Resolves part of EbookFoundation/free-programming-books#5466

* Add DigiKey's RTOS course

Resolves part of EbookFoundation/free-programming-books#5466

books/free-programming-books.md

@@ -1387,6 +1387,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [First Steps with Embedded Systems](https://www.phaedsys.com/principals/bytecraft/bytecraftdata/bcfirststeps.pdf) - Byte Craft Limited (PDF)
 * [Introduction to Embedded Systems, Second Edition](https://ptolemy.berkeley.edu/books/leeseshia/releases/LeeSeshia_DigitalV2_2.pdf) - Edward Ashford Lee, Sanjit Arunkumar Seshia (PDF)
 * [Introduction to Microcontrollers](http://www.embeddedrelated.com/showarticle/453.php) (HTML)
+* [Mastering the FreeRTOS Real Time Kernel - a Hands On Tutorial Guide](https://freertos.org/Documentation/RTOS_book.html) - freertos.org ([PDF](https://freertos.org/fr-content-src/uploads/2018/07/161204_Mastering_the_FreeRTOS_Real_Time_Kernel-A_Hands-On_Tutorial_Guide.pdf))
 
 
 ### Erlang

courses/free-courses-en.md

@@ -569,6 +569,7 @@
 * [MIT's Mathematics for Computer Science](http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-042j-mathematics-for-computer-science-fall-2010/video-lectures/)
 * [Principles of Reactive Programming](https://www.coursera.org/course/reactive)
 * [Robotics I](https://www.youtube.com/playlist?list=PLAQopGWlIcyaqDBW1zSKx7lHfVcOmWSWt) - (A. De Luca)
+* [Shawn Hymel Presents: Introduction to RTOS](https://www.youtube.com/playlist?list=PLEBQazB0HUyQ4hAPU1cJED6t3DU0h34bz) - Digi-Key (Youtube)
 * [Stanford Cryptography I](https://www.coursera.org/course/crypto)
 * [Stanford Cryptography II](https://www.coursera.org/course/crypto2)
 * [Stanford SEE 229 - Machine Learning](https://see.stanford.edu/Course/CS229)