Improve instructions for additional indexes creation

[lbusett]

With reference to this:

Also, if I may make a suggestion, you could add some notes on the syntax to use on a custom index, e.g. the correct names of the bands to be used in order for it to work correctly (in my opinion, simply "b1, ..., b7" would do, but "Red, NIR, ..., SWIR2" would work fine as well). in #51

, improve instructions on additional indexes specifications

[lbusett]

Addressed in 6ee0950 and dc56ff1.

Names of bands available for index creation are now reported in the GUI + improved the README, including more detailed usage instructions