v1.2.1 for ESP8266 using W5x00/ENC28J60 Ethernet

### Releases v1.2.1

1. Initial porting and coding for **ESP8266 using W5x00 or ENC28J60 Ethernet**
2. Bump up version to v1.2.1 to sync with [AsyncUDP_STM32](https://github.com/khoih-prog/AsyncUDP_STM32) v1.2.1

CONTRIBUTING.md

@@ -0,0 +1,53 @@
+## Contributing to AsyncUDP_Ethernet
+
+### Reporting Bugs
+
+Please report bugs in AsyncUDP_Ethernet if you find them.
+
+However, before reporting a bug please check through the following:
+
+* [Existing Open Issues](https://github.com/khoih-prog/AsyncUDP_Ethernet/issues) - someone might have already encountered this.
+
+If you don't find anything, please [open a new issue](https://github.com/khoih-prog/AsyncUDP_Ethernet/issues/new).
+
+### How to submit a bug report
+
+Please ensure to specify the following:
+
+* Arduino IDE version (e.g. 1.8.19) or Platform.io version
+* Board Core Version (e.g. ESP8266 core v3.0.2)
+* Contextual information (e.g. what you were trying to achieve)
+* Simplest possible steps to reproduce
+* Anything that might be relevant in your opinion, such as:
+  * Operating system (Windows, Ubuntu, etc.) and the output of `uname -a`
+  * Network configuration
+
+
+### Example
+
+```
+Arduino IDE version: 1.8.19
+ESP8266_NODEMCU_ESP12E using ESP8266_W5500 Ethernet
+ESP8266 core v3.0.2
+OS: Ubuntu 20.04 LTS
+Linux xy-Inspiron-3593 5.13.0-39-generic #44~20.04.1-Ubuntu SMP Thu Mar 24 16:43:35 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
+
+Context:
+I encountered an endless loop while trying to connect to Local WiFi.
+
+Steps to reproduce:
+1. ...
+2. ...
+3. ...
+4. ...
+```
+### Sending Feature Requests
+
+Feel free to post feature requests. It's helpful if you can explain exactly why the feature would be useful.
+
+There are usually some outstanding feature requests in the [existing issues list](https://github.com/khoih-prog/AsyncUDP_Ethernet/issues?q=is%3Aopen+is%3Aissue+label%3Aenhancement), feel free to add comments to them.
+
+### Sending Pull Requests
+
+Pull Requests with changes and fixes are also welcome!
+