added sinhala courses (#5934)

* added sinhala courses

* updateted free-courses-sl.md

* free-courses-sl.md updated v2

* Update courses/free-courses-sl.md

Update courses/free-courses-sl.md v3

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update courses/free-courses-sl.md

Update courses/free-courses-sl.md v4

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update courses/free-courses-sl.md v5

Update courses/free-courses-sl.md v5

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update courses/free-courses-sl.md

Update courses/free-courses-sl.md v6

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update courses/free-courses-sl.md

Update courses/free-courses-sl.md v6

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update courses/free-courses-sl.md

Update courses/free-courses-sl.md v7

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update courses/free-courses-sl.md

Update courses/free-courses-sl.md v8

* updated free-courses-sl.md 

resolved empty catogory

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

README.md

@@ -94,6 +94,7 @@ Please read [CONTRIBUTING](CONTRIBUTING.md). If you're new to GitHub, [welcome](
 + [Portuguese (Brazil)](courses/free-courses-pt_BR.md)
 + [Portuguese (Portugal)](courses/free-courses-pt_PT.md)
 + [Russian / Русский язык](courses/free-courses-ru.md)
++ [Sinhala / සිංහල](courses/free-courses-sl.md)
 + [Spanish / español / castellano](courses/free-courses-es.md)
 + [Thai / ภาษาไทย](courses/free-courses-th.md)
 + [Turkish / Türkçe](courses/free-courses-tr.md)