Add docs about debugging (#37)

* Add CODEOWNERS

* Add doc for debug

* Update readme

* Update readme

* Update docs/debugging.md

Co-authored-by: Adrian Pilarczyk <adrianpilarczyk314@gmail.com>

* Update docs/debugging.md

Co-authored-by: Adrian Pilarczyk <adrianpilarczyk314@gmail.com>

* Update docs/debugging.md

Co-authored-by: Adrian Pilarczyk <adrianpilarczyk314@gmail.com>

* Apply suggestions from code review

Co-authored-by: Adrian Pilarczyk <adrianpilarczyk314@gmail.com>

* Add info about debug in iframe

Co-authored-by: Adrian Pilarczyk <adrianpilarczyk314@gmail.com>

CODEOWNERS

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+* @saleor/devtools

README.md

@@ -24,6 +24,8 @@ npm i @saleor/app-sdk
 ## Docs
 
 - [AppBridge](./docs/app-bridge.md) - communication layer between Saleor App and Dashboard
+- [APL](./docs/apl.md) - an interface that enabled App communicating between one or more Saleor instances
+- [Debugging](./docs/debugging.md) - how to debug app-sdk behaviour
 
 ## Development
 

docs/CODEOWNERS

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+* @saleor/docs

docs/debugging.md

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+# Debugging
+
+The ability to debug is important for app developers.
+App SDK provides opt-in verbose logs, that can be enabled for specific modules.
+
+Debug logs are implemented with [debug](https://www.npmjs.com/package/debug) package and work both in Node and Browser.
+
+## Enabling verbose logs
+
+Debug logs are aggregated within [namespaces](#namespaces). They can be enabled, disabled, or configured.
+
+By default, all verbose logs are disabled.
+
+To enable logs, use the following:
+
+### Enabling in Node context
+
+Add `DEBUG={namespaces}` env variable.
+
+#### Example
+
+```shell
+DEBUG=* pnpm run dev
+```
+
+For more details check [debug package](https://github.com/debug-js/debug#usage).
+
+### Enabling in Browser context
+
+Set `localStorage.debug = '{namepsaces}'` in your frontend code. For more details see [debug package](https://github.com/debug-js/debug#browser-support).
+
+#### Debug in iframe
+
+Note, that Saleor Apps are usually hosted on a different domain than Saleor Dashboard.
+That means `localStorage` must be called in the context of the App's URL, not Saleor Dashboard.
+
+#### Example
+
+```javascript
+// In devtools console
+localStorage.debug = "*";
+```
+
+## Namespaces
+
+Use the namespace name to enable debug logs for each module.
+
+| Namespace name | Description                                        |
+| -------------- | -------------------------------------------------- |
+| \*             | Enable all                                         |
+| AppBridge      | Enable [AppBridge](./app-bridge.md) (browser only) |