Update Watchdog.md

Create Watchdog contributing API in 5.8.

Author: Amanda Butler

docs/reference/contributing/target/Watchdog.md

@@ -1,25 +1,49 @@
 <h2 id="watchdog-port">Watchdog</h2>
 
-[Include a brief description here.]
+The Watchdog HAL API provides a low-level interface to the Independent Watchdog Timer of a target. Implementing the Watchdog API is not mandatory, but by implementing it, you can use the hardware watchdog timer to detect and recover from computer malfunctions.
+
+Implement the ResetReason API when implementing the Watchdog API. The ResetReason API allows you to detect at boot time whether a watchdog caused the last reset.
 
 ### Assumptions
 
 #### Defined behavior
 
-[Include any defined behavior in bullet format here.]
+- Sleep and debug modes don't stop the watchdog timer from counting down.
+- The function `hal_watchdog_init` is safe to call repeatedly. The function's implementation must not do anything if `hal_watchdog_init` has already initialized the hardware watchdog timer.
+- Maximum supported timeout is `UINT32_MAX` milliseconds; minimum timeout is 1ms.
+- The timeout must be accurate to the nearest millisecond.
 
 #### Undefined behavior
 
-[Include any undefined behavior in bullet format here.]
+- Calling any function other than `hal_watchdog_init` or `hal_watchdog_get_platform_features` before you have initialized the watchdog.
+
+#### Notes
+
+- A software reset may not stop the watchdog timer; the behavior is platform specific.
+- Timing on most platforms is based on the timeout registers and a prescaler value. They should be accurate to the nearest millisecond but may be off my a several µs.
 
-#### Potential bugs
+### Dependency
 
-[Include any potential bugs in bullet format here.]
+Hardware Independent Watchdog support.
 
 ### Implementing Watchdog
 
-[Include implementation information here.]
+You can find the API and specification for the Watchdog API in the following header file:
+
+[![View code](https://www.mbed.com/embed/?type=library)](http://os.mbed.com/docs/v5.8/feature-hal-spec-watchdog-doxy/classmbed_1_1_watchdog.html)
+
+To enable Watchdog support in Mbed OS, add the `WATCHDOG` label in the `device_has` option of the target's section in the `targets.json` file.
 
 ### Testing
 
-[Include testing information here.]
+The Mbed OS HAL provides a set of conformance tests for Watchdog. You can use these tests to validate the correctness of your implementation. To run the Watchdog HAL tests use the following command:
+
+```
+mbed test -t <toolchain> -m <target> -n "tests-mbed_hal-watchdog*"
+```
+
+You can read more about the test cases here:
+
+[![View code](https://www.mbed.com/embed/?type=library)](http://os.mbed.com/docs/v5.8/feature-hal-spec-watchdog-doxy/watchdog__api__tests_8h_source.html)
+
+[![View code](https://www.mbed.com/embed/?type=library)](http://os.mbed.com/docs/v5.8/feature-hal-spec-watchdog-doxy/watchdog__reset__tests_8h_source.html)