Add Flattening nested arrays Info with Example

[Swap76]

Create a new file named Flattening in JavaScript_Basics and add the following:

• Where we use the Flattening

• More info if there about Flattening

• Create .md and .js both files as we are shifting to Docsify

[eduaugustus]

I can do this!

[Swap76]

hey @eduaugustus some one has already created a pull request