v2.1.0 to fix multiple-definitions linker error

#### Releases v2.1.0

1. Fix multiple-definitions linker error. Check [Multiple definitions #1](#1)
2. Add example [multiFileProject](examples/multiFileProject) to demo for multiple-file project to avoid `multiple-definitions` linker error

Author: khoih-prog

README.md

@@ -307,7 +307,7 @@ void loop()
 
 #### File [AsyncUdpNTPClient.ino](examples/AsyncUdpNTPClient/AsyncUdpNTPClient.ino)
 
-https://github.com/khoih-prog/AsyncUDP_Teensy41/blob/79a2019f3482e6902406bacffd483bcdf6921195/examples/AsyncUdpNTPClient/AsyncUdpNTPClient.ino#L12-L199
+https://github.com/khoih-prog/AsyncUDP_WT32_ETH01/blob/38f14694c93ca0a393adfeffaee0027924a7e545/examples/AsyncUdpNTPClient/AsyncUdpNTPClient.ino#L11-L175
 
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