Add linux file and parallel ref

docs/cheatsheet.md

@@ -1,3 +1,5 @@
+[Overview](readme.md)
+
 This is intended as a quick reference and showcase. For more complete info, see [John Gruber's original spec](http://daringfireball.net/projects/markdown/) and the [Github-flavored Markdown info page](http://github.github.com/github-flavored-markdown/).
 
 Note that there is also a [Cheatsheet specific to Markdown Here](./Markdown-Here-Cheatsheet) if that's what you're looking for. You can also check out [more Markdown tools](./Other-Markdown-Tools).

docs/linux.md

@@ -0,0 +1,7 @@
+[Overview](../README.md)
+
+- [Shell](#shell)
+
+## Shell
+
+[Parallel running and waiting](https://hackaday.com/2021/08/16/linux-fu-walk-chew-gum/)

docs/php.md

@@ -1,3 +1,5 @@
+[Overview](../README.md)
+
 - [Symfony](#symfony)
 - [Fun](#fun)
 

docs/readme.md

@@ -1,3 +1,6 @@
-[Cheat sheet](cheatsheet.md)
+[Back](../README.md)
 
-[PHP](php.md)
+## Table of contents
+- [Cheat sheet](cheatsheet.md)
+- [PHP](php.md)
+- [Linux](linux.md)