Update free-programming-playgrounds.md to add some online compiler links (#9733)

* Update free-programming-playgrounds.md

* Update free-programming-playgrounds.md

more/free-programming-playgrounds.md

@@ -72,6 +72,7 @@
 
 ### <a id="csharp"></a>C\#
 
+* [OnlineGDB](https://www.onlinegdb.com/online_csharp_compiler)
 * [Programiz - Online Compiler](https://www.programiz.com/csharp-programming/online-compiler/)
 * [SoloLearn](https://code.sololearn.com/csharp)
 
@@ -171,6 +172,7 @@
 * [InterviewBit - Online Java Compiler IDE](https://www.interviewbit.com/online-java-compiler/)
 * [JDoodle - Online Java Compiler Advanced IDE](https://www.jdoodle.com/online-java-compiler-ide/)
 * [JDoodle - Online Java Compiler IDE](https://www.jdoodle.com/online-java-compiler/)
+* [OnlineGDB](https://www.onlinegdb.com/online_java_compiler)
 * [Programiz - Online Java Compiler](https://www.programiz.com/java-programming/online-compiler/)
 * [repl.it](https://repl.it) (_including a separate Java/Swing_)
 * [SoloLearn](https://code.sololearn.com/java)
@@ -271,6 +273,7 @@
 
 ### R
 
+* [myCompiler](https://www.mycompiler.io/online-r-compiler)
 * [R-Fiddle](http://www.r-fiddle.org)
 * [Rextester](https://rextester.com/l/r_online_compiler)
 * [SoloLearn](https://code.sololearn.com/r)