Include deg2rad constant in README.md

[tylerdiaz]

This variable is referenced when initializing the observerGd object in the example code.

[coveralls]

Coverage remained the same at 81.168% when pulling 69d7d8a on tylerdiaz:patch-1 into 7b00ec4 on shashwatak:develop.

[ezze]

@tylerdiaz, thanks for PR.
I think that defining deg2rad constant in such way in the examples is not best solution. I propose to use degreesToRadians function instead. In the case of UMD its usage will look like this:

var observerGd = {
    longitude: satellite.degreesToRadians(-122.0308),
    latitude: satellite.degreesToRadians(36.9613422),
    height: 0.370
};

In a case of ES modules one should import the function this way:

import { degreesToRadians } from 'satellite.js';

[thkruz]

I think either option is better than the current example. @ezze's method makes it more modular.

[ezze]

Updated in README.md in 5753e39.

[pkrasicki]

It would be good to mention some constants in readme. Things like earthRadius might be useful for calculations.