Thanks to @kroimon for the heads up!

The current version of ESPHome shows a warning about GPIO4:
```
WARNING GPIO4 is a Strapping PIN and should be avoided.
Attaching external pullup/down resistors to strapping pins can cause unexpected failures.
See https://esphome.io/guides/faq.html#why-am-i-getting-a-warning-about-strapping-pins
```
The warning can be ignored, so I added a remark about this to the documentation.
Thanks to @jossuanet for the heads up about this warning.

Same fix as already applied for release/2021.10.0b1.

Made the lamp work with ESPHome 2021.10.0

Thanks to @maartenpaul for the heads up!

- Fixed transitioning from the off to on state. Thanks to @marcel1988 for informing
  me about his non-functional wake-up light script. Before this fix, the light would
  stay off during the full transformation, only to turn on at the very end.
- The `on_brightness` trigger is now also called at the start of a transition.
  This fixes a problem with the brightness slider illumination not following along
  when you slide your finger over it.
- Suppressing a transition time for transitioning to a night light color is now
  only instant when starting from a night light setting. Before this change, moving
  from for example 80% brightness to 1% brightness (i.e. night light), would be
  instant. Now an actual transformation is done.

* New light transformer implementation
* Fixed missing ; in the code, thanks Jos!
* Removed backward compatibility code for v1.20.x (because there were too many other changes that broke the build, and for which creating a work-around was not feasible).
* Fixed duplicate api section in the example.yaml (thanks @badbroechten\!) and updated the example to not use an underscore in the hostname.
* Prepare for release

* Validate preset names and group names at compile time (fixes #15)

* Small fix for using a less confusing var name.

Co-authored-by: Maurice Makaay <mmakaay1@xs4all.net>