Add content to creating custom pipe, filters, directives and libraries (#2320)

* Added content to readme.md file of 112-creating-a-custom-x folder

[#1847 ](https://github.com/kamranahmedse/developer-roadmap/issues/1847) I made the required addition of content to readme.md file of 112-creating-a-custom-x to resolve the issue.

* Update content/roadmaps/104-angular/content/112-creating-a-custom-x/readme.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/104-angular/content/112-creating-a-custom-x/readme.md

@@ -1 +1,3 @@
-# Creating a custom x
+# Creating a custom X
+
+Learn how to create custom pipes, libraries and directives in Angular.