Merge branch 'doc/add_coap_readme' into 'release/v3.2'

doc: add CoAP README.md(backport v3.2)

See merge request idf/esp-idf!4262

examples/protocols/coap_client/README.md

@@ -0,0 +1,65 @@
+
+# CoAP client example
+
+(See the README.md file in the upper level 'examples' directory for more information about examples.)
+this CoAP client example is adaptation of one of the [libcoap](https://github.com/obgm/libcoap) example.
+
+CoAP client example would connect your ESP32 device to any CoAP server, fetch data from CoAP server and upstream data to CoAP server.
+
+The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained networks in the Internet of Things.   
+The protocol is designed for machine-to-machine (M2M) applications such as smart energy and building automation.
+
+Please refer to [RFC7252](https://www.rfc-editor.org/rfc/pdfrfc/rfc7252.txt.pdf) for more details.
+
+## How to use example
+
+### Configure the project
+
+```
+make menuconfig
+```
+
+* Set serial port under Serial Flasher config
+* Set Target Uri under Example Configuration
+* Set WiFi SSID under Example Configuration
+* Set WiFi Password under Example Configuration
+
+### Build and Flash
+
+Build the project and flash it to the board, then run monitor tool to view serial output:
+
+```
+make -j4 flash monitor
+```
+
+(To exit the serial monitor, type ``Ctrl-]``.)
+
+See the Getting Started Guide for full steps to configure and use ESP-IDF to build projects.
+
+## Example Output
+Prerequisite: we startup a CoAP server on coap server example.  
+
+and you could receive data from CoAP server if succeed,  
+such as the following log:
+
+```
+...
+I (332) wifi: mode : sta (30:ae:a4:04:1b:7c)
+I (1672) wifi: n:11 0, o:1 0, ap:255 255, sta:11 0, prof:1
+I (1672) wifi: state: init -> auth (b0)
+I (1682) wifi: state: auth -> assoc (0)
+I (1692) wifi: state: assoc -> run (10)
+I (1692) wifi: connected with huawei_cw, channel 11
+I (1692) wifi: pm start, type: 1
+
+I (2582) event: sta ip: 192.168.3.89, mask: 255.255.255.0, gw: 192.168.3.1
+I (2582) CoAP_client: Connected to AP
+I (2582) CoAP_client: DNS lookup succeeded. IP=192.168.3.84
+Received: Hello World!
+E (8102) CoAP_client: select timeout
+E (13102) CoAP_client: select timeout
+...
+```
+
+## Troubleshooting
+* Please make sure Target Url includes valid `host`, `port`, `path`, and begins with `coap://`  

examples/protocols/coap_server/README.md

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+
+# CoAP server example
+
+(See the README.md file in the upper level 'examples' directory for more information about examples.)  
+This CoAP server example is adaptation of one of the [libcoap](https://github.com/obgm/libcoap) example.
+
+CoAP server example would startup a daemon task, receive data from CoAP client and transmit data to CoAP client.
+
+The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained networks in the Internet of Things.   
+The protocol is designed for machine-to-machine (M2M) applications such as smart energy and building automation.
+
+Please refer to [RFC7252](https://www.rfc-editor.org/rfc/pdfrfc/rfc7252.txt.pdf) for more details.
+
+## How to use example
+
+### Configure the project
+
+```
+make menuconfig
+```
+
+* Set default serial port under Serial Flasher config
+* Set WiFi SSID under Example Configuration
+* Set WiFi Password under Example Configuration
+
+### Build and Flash
+
+Build the project and flash it to the board, then run monitor tool to view serial output:
+
+```
+make -j4 flash monitor
+```
+
+(To exit the serial monitor, type ``Ctrl-]``.)
+
+See the Getting Started Guide for full steps to configure and use ESP-IDF to build projects.
+
+## Example Output
+current CoAP server would startup a daemon task,   
+and the log is such as the following:  
+
+```
+...
+I (332) wifi: mode : sta (30:ae:a4:04:1b:7c)
+I (1672) wifi: n:11 0, o:1 0, ap:255 255, sta:11 0, prof:1
+I (1672) wifi: state: init -> auth (b0)
+I (1682) wifi: state: auth -> assoc (0)
+I (1692) wifi: state: assoc -> run (10)
+I (1692) wifi: connected with huawei_cw, channel 11
+I (1692) wifi: pm start, type: 1
+
+I (2622) event: sta ip: 192.168.3.84, mask: 255.255.255.0, gw: 192.168.3.1
+I (2622) CoAP_server: Connected to AP
+E (7622) CoAP_server: select timeout
+E (12622) CoAP_server: select timeout
+E (17622) CoAP_server: select timeout
+...
+```
+
+if a CoAP client query `/Espressif` resource, CoAP server would return `"Hello World!"`  
+
+## Troubleshooting
+* Please make sure CoAP client fetchs data under path: `/Espressif`