Change some instructions in the README.md (#573)
Change some instructions in the README.md (#573)

* Change git commands in the README.md for beginners

* Only clone PLFA shallowly.
* Correct the version of stdandard library for PLFA.

* Install `fix-whitespace` via Stackage

The program `fix-whitespace` is available on Stackage already.
wenkokke/sf,wenkokke/sf