Elm language resources added (EbookFoundation#3054)

* added Solidity

* added Solidity

* added Solidity

* bitcoin book added

* fixed ordering on bitcoin book entry

* added authors for bitcoin and solidity books

* added Bitcoin and cryptocurrency Princeton book

* elm interactive playground added (ellie)

* free elm books added

* elm podcast added

* corrected spacing

* reordered elm books into existing category

* reordered format and author

* removed trailing /s

* fixed alphabetical ordering

Author: alcole

free-podcasts-screencasts-en.md

@@ -16,6 +16,7 @@
 * [Haskell](#haskell)
 * [Javascript](#javascript)
   * [Angular.js](#angularjs)
+  * [Elm](#elm)
   * [Ember.js](#emberjs)
   * [Node.js](#nodejs)
   * [React.js](#reactjs)
@@ -143,6 +144,11 @@
 * [Angular Air](https://angularair.com) (podcast)
 
 
+#### Elm
+
+* [Elm Town](https://elmtown.audio) (podcast)
+
+
 #### Ember.js
 
 * [Ember Weekend](https://emberweekend.com/episodes) (podcast)

free-programming-books.md

@@ -39,6 +39,7 @@
 * [Elixir](#elixir)
   * [Ecto](#ecto)
   * [Phoenix](#phoenix)
+* [Elm](#elm)  
 * [Emacs](#emacs)
 * [Embedded Systems](#embedded-systems)
 * [Erlang](#erlang)
@@ -1559,9 +1560,13 @@ Kerridge (PDF) (email address *requested*, not required)
 
 #### Elm
 
+* [An Introduction to Elm](https://guide.elm-lang.org) (HTML)
+* [Beginning Elm](https://elmprogramming.com) - Pawan Poudel  (HTML)
 * [Building a Live-Validating Signup Form in Elm](http://tech.noredink.com/post/129641182738/building-a-live-validated-signup-form-in-elm)
 * [Elm Accelerated](https://accelerated.amimetic.co.uk) - James Porter
+* [Elm Programming Language](https://en.wikibooks.org/wiki/Elm_programming_language) (HTML)
 * [Elm Tutorial](http://www.elm-tutorial.org)
+* [Learn You an Elm](https://learnyouanelm.github.io) (HTML)
 * [The Elm Architecture](https://github.com/evancz/elm-architecture-tutorial)
 
 

free-programming-playgrounds.md

@@ -4,6 +4,7 @@
 * [ClojureScript](#clojurescript)
 * [Crystal](#crystal)
 * [CSS](#css)
+* [Elm](#elm)
 * [FlexBox](#flexbox)
 * [Go](#go)
 * [Haskell](#haskell)
@@ -45,6 +46,11 @@
 * [Dabblet](http://dabblet.com)
 
 
+### Elm
+
+* [Ellie](https://ellie-app.com)
+
+
 ### FlexBox
 
 * [Echoplex](http://the-echoplex.net/flexyboxes/)