Add Mostly adequate guide to "Functional" section

[nelsonic]

Todo

• [ ] add Mostly adequate guide to FP (in javascript) https://github.com/MostlyAdequate/mostly-adequate-guide to the functional section: https://github.com/dwyl/learn-tdd#functional

[luqmanoop]

Link to the guide should be added under the "functional" section, right?

[nelsonic]

@codeshifu yeah, feels like a good thing to add to the list of bullet points:

Also spotted a typo:

The "functional" soltion

should be:

The "functional" solution

[luqmanoop]

@nelsonic Aiight. Will send a PR soon.

[nelsonic]

@codeshifu thanks for adding! 🎉