Added ESP8266 support and documentation.

[Spoozy]

Q	A
Bug fix?	no
New feature?	yes
Doc update?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	no

[Rotzbua]

Hi, thanks for your PR.
A full list of pin configs is available at: https://github.com/miguelbalboa/rfid#pin-layout
Updating every example with new pin layouts for a lot of board is a bit overdocumentation. Maybe it would be useful to have a link to the readme in every example.

[Spoozy]

No worries, although I could change the documentation, the most important thing is to have the constexpr uint8_t RST_PIN = D3; as the normal #define RST_PIN 9 doesn't work using the ESP8266 from what I have tested. I will implement the link to the README :)

[hochwasser]

What is the problem when the define got replaced with const or sonstexpr? Does this break old code?
If not, I would suggest replacing the defines.

[Rotzbua]

@hochwasser The problem is that all tutorials and instructions with this library uses #defines. The usage of constexpr would not break anything but confuses Arduino beginners.

I created a fork #522 which targets new concepts.

[Rotzbua]

@Spoozy the pr conflicts so I added the hint the link to the readme to every example. I think that is the initial intention of the pr. Thanks for contribution 👍

[hochwasser]

@Rotzbua I understand that constexpr is a relatively new. const could also be used in this situation.