Add "not recommended" note to CC2530 and C2531

[Hedda]

Add a "not recommended for Zigbee networks with more than 20 devices" note to both CC2530 and C2531 in README.md

ZHA / zigpy developers elected to not whitelist "TI CC2531" USB adapter ZHA discovery due to its older less powerful hardware:

home-assistant/core#55298

Zigbee2MQTT as well added "not recommended" notes to both CC2530 and CC2531 in their official supported adapters list:

https://www.zigbee2mqtt.io/information/supported_adapters

• Texas Instruments CC2531 (not recommended)
• Texas Instruments CC2530 (not recommended)

https://www.zigbee2mqtt.io/information/supported_adapters#notes

Adapters based on the CC2530 or CC2531 chip are not powerful and not recommended for networks larger than 20 devices.

Also, Zigbee2MQTT / zigbee-herdsman developer Koenkk noted Z-Stack 3.0.x firmware is not recommended on C2530/C2531:

https://github.com/Koenkk/Z-Stack-firmware/tree/master/coordinator#im-using-a-cc2530-or-cc2531-which-firmware-should-i-use

I'm using a CC2530 or CC2531, which firmware should I use?

This depends:

• Zigbee 3.0 firmwares are not recommended for the CC2530 and CC2531 (since those are not powerful enough)
• If you have a network of 1 - 15 devices, the Z-Stack_Home_1.2 default firmware is recommended.
• If you have a network of 15+ devices, the Z-Stack_Home_1.2 source routing firmware is recommended.
• Note that the source routing firmware only supports 5 direct children, therefore you need to have routers in range of the coordinator.

[codecov-commenter]

Codecov Report

Merging #87 (380c58f) into dev (eff5c0c) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev      #87      +/-   ##
==========================================
- Coverage   98.71%   98.68%   -0.03%     
==========================================
  Files          44       44              
  Lines        3727     3727              
==========================================
- Hits         3679     3678       -1     
- Misses         48       49       +1     

Impacted Files	Coverage Δ
zigpy_znp/zigbee/application.py	95.74% <0.00%> (-0.18%)	⬇️

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[puddly]

Thanks!