Merge pull request #1415 from SuperStas/http2_explained_en

Added "HTTP2 Explained" book (EN)

free-programming-books.md

@@ -468,6 +468,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [Code Connected vol.1](http://hintjens.wdfiles.com/local--files/main%3Afiles/cc1pe.pdf) (PDF) (book on ZeroMQ)
 * [High-Performance Browser Networking](http://chimera.labs.oreilly.com/books/1230000000545/index.html)
 * [HTTP Succinctly, Syncfusion](http://www.syncfusion.com/resources/techportal/ebooks/http) (PDF, Kindle) *(Just fill the fields with any values)*
+* [HTTP2 Explained](http://daniel.haxx.se/http2/) - Daniel Stenberg
 * [The TCP/IP Guide](http://www.tcpipguide.com/free/t_toc.htm)
 * [Understanding IP Addressing: Everything you ever wanted to know](http://www.apnic.net/__data/assets/pdf_file/0020/8147/501302.pdf) (PDF)
 * [ZeroMQ Guide](http://zguide.zeromq.org/page%3Aall)