Update README.md (added information regarding custom endpoints)

[HanEmile]

After hassling with my custom endpoint for some time, I realized that it did not utilize https and hadn't set the Access-Control-Allow-Origin endpoint resulting in no values drawn on the map. In order to help people who might have the same problem in the future, I've added that information in the custom endpoint section.

[CLAassistant]

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[HanEmile]

signed the CLA