added the solidity programming language for writing smart contracts i… (#5995)

* added the solidity programming language for writing smart contracts in Ethereum

* updated the spaces between the previous and the next sections

* Added few more resources for solidity development which includes step by step guide

* Fixed the alphabetical order

* removed the specific version based on the review comment

* removed the course in the solidity section to have it just books and guides

books/free-programming-books-langs.md

@@ -1021,6 +1021,12 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [The JasperReports Ultimate Guide, Third Edition](http://jasperreports.sourceforge.net/JasperReports-Ultimate-Guide-3.pdf) (PDF)
 
 
+#### Solidity
+
+* [Introductory guide for Solidity](https://www.tutorialspoint.com/solidity/index.htm)
+* [The Solidity Reference Guide](https://docs.soliditylang.org)
+
+
 #### Spring
 
 * [Building Applications with Spring 5 and Vue.js 2](https://www.packtpub.com/free-ebooks/building-applications-spring-5-and-vuejs-2) - James J. Ye (Packt account *required*)