Clarify contribution instructions

Since the previous "better than it was before..." PR of mine showed the usage of sed, and Seth pointed out w/o merge that "Sed doesn't exist on Windows", I have gone through hoops to:

1. Close the other PR since it was made via "Edit" on GH
2. Change the *EXISTING UNIX/Linux-specific instructions mentioning sudo* (*AHEM*) to show how to do things on both Windows and UNIX/Linux.
3. Kept sed out of the information and suggested people examine the JSON file however they see fit (but to examine it).

Hopefully this will satiate everyone's extremely stringent requirements for utmost precision community contributions to a README prior to merge, in order to save others the 2 hours I wasted chasing this down.

:|