Create examples.md

[arouinfar]

For #180

• Derived strings are not autoselectable, see Instrument PatternStringProperty and DerivedProperty for strings that need to be autoselectable in Studio. #186. This means the characters used for coefficients and variables are not discoverable with autoselect. There may be different conventions for labeling these quantities, so clients could want to customize these strings. Include an example explaining how to customize a, b, c, h, k, etc.

[pixelzoom]

@arouinfar If this is not completed before dev testing begins in phetsims/qa#959, please provide QA with instructions on what they should do about examples.md.

[arouinfar]

Examples updated in the above commits, closing.