Add documentation about defining custom operators

[nlassaux]

Many extensions define new operators, for example to compute various distances between geometries, vectors, etc. A useful example is <-> being used by PostGIS.

Added the documentation explaining how to leverage ArithmeticExpression to extend pypika with it, right under the explanation about how to define a custom function. Having that doc section would have saved me quite some time actually.

Thanks @wd60622 for explaining the approach!

[wd60622]

Looks awesome. Thanks for making the adjustment

Only thing I would suggest is adding a brief section on this in the README like with the Custom Functions section

Any thoughts @AzisK ?

[AzisK]

Thanks to both of you. I don't have any additional suggestions except writing a test to be sure that people actually can define their own custom operators

[wd60622]

Thanks to both of you. I don't have any additional suggestions except writing a test to be sure that people actually can define their own custom operators

Think that's fair. If someones going into the code base, I think ideally someone might be able to use a string instead of a small class.