docs: update readme for windows

[dkocich]

According to http://postgrest.org/en/v6.0/tutorials/tut0.html i used setx /m PATH "%PATH%;C:\Program Files\PostgreSQL\10\bin" to adjust my path but terminal

λ setx /m PATH "%PATH%;C:\Program Files\PostgreSQL\10\bin"
WARNING: The data being saved is truncated to 1024 characters.
SUCCESS: Specified value was saved.

(possible alternative https://stackoverflow.com/a/37304698/4114514)

This messed my path variable. It should be mentioned in docs otherwise it will do the same thing for others on windows too...

[steve-chavez]

Oh, that's not good. Thanks for reporting.

Adding the warning and the powershell alternative on the note would be good, you could submit a PR if you'd like to help us.

(moving the issue to docs)