Add book and section on ESP8266 (#2126)

* add ESP8266 section and a book to it

* fix link case

free-programming-books.md

@@ -38,6 +38,7 @@
 * [Emacs](#emacs)
 * [Embedded Systems](#embedded-systems)
 * [Erlang](#erlang)
+* [ESP8266](#esp8266)
 * [F#](#f-sharp)
 * [Firefox OS](#firefox-os)
 * [Force.com](#forcecom)
@@ -1015,6 +1016,11 @@
 * [Stuff Goes Bad: Erlang in Anger](https://s3.amazonaws.com/erlang-in-anger/text.v1.0.3.pdf) - Fred Herbert (PDF)
 
 
+### ESP8266
+
+* [Kolban's book on the ESP32 & ESP8266](https://leanpub.com/ESP8266_ESP32)
+
+
 ### F Sharp
 
 * [F# for fun and profit](https://www.gitbook.com/book/swlaschin/fsharpforfunandprofit/details) (ePub)