Reads the contents of a file as utf-8 and returns the contents as a string. The filename is relative to the location that is specified (this is a flat directory).
Read-only location parameters:
app The app's own directory (so you can read files included in the app's zip file).
Read and write location parameters:
nv Non-volatile storage. Data stored here survives firmware updates.
Name of the file to act on.
Using digium.readFile / digium.writeFile to store settings.