added Mastering Ethereum in Blockchain category (#6735)

* added Mastering Ethereum in Blockchain category

* Update books/free-programming-books-subjects.md for normalization

Co-authored-by: David Ordás <[email protected]>

Co-authored-by: David Ordás <[email protected]>

Author: SerafDosSantos

Diff for: books/free-programming-books-subjects.md

@@ -143,7 +143,8 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Getting Started with Enterprise Blockchain: A Guide to Design and Development](https://www.ibm.com/downloads/cas/RYWXAR0M) - Michael Bradley, David Gorman, Matt Lucas, Matthew Golby-Kirk (PDF)
 * [Grokking Bitcoin](https://rosenbaum.se/book/) - Kalle Rosenbaum (HTML)
 * [IBM Blockchain: The Founder’s Handbook, Third Edition](https://www.ibm.com/downloads/cas/GZPPMWM5) - Antonio Banda, Matthew Hamilton, Eileen Lowry, John Widdifield et al. (PDF)
-* [Mastering Bitcoin - Unlocking digital currencies](https://github.com/bitcoinbook/bitcoinbook) - Andreas M. Antonopoulos
+* [Mastering Bitcoin - Unlocking digital currencies (2017), 2nd Edition](https://github.com/bitcoinbook/bitcoinbook) - Andreas M. Antonopoulos (AsciiDoc)
+* [Mastering Ethereum (2018), 1st Edition](https://github.com/ethereumbook/ethereumbook) - Andreas M. Antonopoulos, Gavin Wood (AsciiDoc)
 * [Playtime with Hyperledger Composer](https://schadokar.dev/ebooks/playtime-with-hyperledger-composer/) - Shubham Chadokar (PDF)