Added: Free courses and Resource in khmer language. (#6156)

* Added: Free courses and Resource in khmer language.

Added: Free courses and Resource in khmer language.

* Fixed: linter error.

Fixed: linter error.

* Fixed: build error

* Added: khmer language  course to readme. 

Added: khmer language  course to readme.

* Changed: khmer iso code from kh to km

* changed: khmer iso code from kh to km

* Changed content and fix links.

Removed non khmer courses, changed link youtube, and  fix Alphabetized order

* fixed: order alphatically

* fixed: title and content error.

* removed: non khmer courses.

README.md

@@ -89,6 +89,7 @@ Please read [CONTRIBUTING](CONTRIBUTING.md). If you're new to GitHub, [welcome](
 + [Indonesian / Bahasa Indonesia](courses/free-courses-id.md)
 + [Italian / italiano](courses/free-courses-it.md)
 + [Kazakh / қазақша](courses/free-courses-kk.md)
++ [Khmer / ភាសាខ្មែរ](courses/free-courses-km.md)
 + [Korean / 한국어 [韓國語]](courses/free-courses-ko.md)
 + [Persian / Farsi (Iran) / فارسى](courses/free-courses-fa_IR.md)
 + [Polish / polski / język polski / polszczyzna](courses/free-courses-pl.md)

courses/free-courses-km.md

+### មាតិកា
+
+* [Computer Science](#computer-science)
+* [Flutter](#flutter)
+* [Git](#git)
+* [Javascript](#javascript)
+* [Web Development](#web-development)
+
+
+### Computer Science
+
+* [ចំនេះដឹងទូទៅ](https://youtube.com/playlist?list=PLB5U9f77LXqL-IC2MAoaKl1tJOuiQZbZQ) - TFD
+
+
+### Flutter
+
+* [Flutter food ordering app](https://youtube.com/playlist?list=PL9nDNu0HsFZk6qC7nfhdYbnB-B9wyfKV9) - Chunlee Thong 
+* [Flutter UI Speed Code](https://youtube.com/playlist?list=PLVY9IbkulBUiKDrT5BFcMKXxtk4b0IJIX) - Sopheaman Van
+
+
+### Git
+
+* [Git](https://youtube.com/playlist?list=PLyNTduYoTjqBsCRtQrkUw-jaBLsInhsJa) - Soeng Souy
+
+
+### Javascript
+
+* [មេរៀន Javascript Speak khmer](https://youtube.com/playlist?list=PLWrsrLN26mWZiRcn4O-cphCw-AyoWumhq) - រៀនIT
+
+
+### Web Development
+
+* [👨‍💻👨‍💻 Coding](https://youtube.com/playlist?list=PLxchvQVIj9rb8O10g494z9EQ0HZO-aU_6) - Sambat Lim