chore(docs): add fetch adapter docs; (#6407)

README.md

@@ -125,6 +125,7 @@
   - [🆕 Progress capturing](#-progress-capturing)
   - [🆕 Rate limiting](#-progress-capturing)
   - [🆕 AxiosHeaders](#-axiosheaders)
+  - [🔥 Fetch adapter](#-fetch-adapter)
   - [Semver](#semver)
   - [Promises](#promises)
   - [TypeScript](#typescript)
@@ -481,10 +482,13 @@ These are the available config options for making requests. Only the `url` is re
   withCredentials: false, // default
 
   // `adapter` allows custom handling of requests which makes testing easier.
-  // Return a promise and supply a valid response (see lib/adapters/README.md).
+  // Return a promise and supply a valid response (see lib/adapters/README.md)
   adapter: function (config) {
     /* ... */
   },
+  // Also, you can set the name of the built-in adapter, or provide an array with their names
+  // to choose the first available in the environment
+  adapter: 'xhr' // 'fetch' | 'http' | ['xhr', 'http', 'fetch']
 
   // `auth` indicates that HTTP Basic auth should be used, and supplies credentials.
   // This will set an `Authorization` header, overwriting any existing
@@ -1289,6 +1293,7 @@ Sending `Blobs`/`Files` as JSON (`base64`) is not currently supported.
 ## 🆕 Progress capturing
 
 Axios supports both browser and node environments to capture request upload/download progress.
+The frequency of progress events is forced to be limited to `3` times per second.
 
 ```js    
 await axios.post(url, data, {
@@ -1609,6 +1614,30 @@ The following shortcuts are available:
 
 - `setContentEncoding`, `getContentEncoding`, `hasContentEncoding`
 
+## 🔥 Fetch adapter
+
+Fetch adapter was introduced in `v1.7.0`. By default, it will be used if `xhr` and `http` adapters are not available in the build,
+or not supported by the environment.
+To use it by default, it must be selected explicitly:
+
+```js
+const {data} = axios.get(url, {
+  adapter: 'fetch' // by default ['xhr', 'http', 'fetch']
+})
+```
+
+You can create a separate instance for this:
+
+```js
+const fetchAxios = axios.create({
+  adapter: 'fetch'
+});
+
+const {data} = fetchAxios.get(url);
+```
+
+The adapter supports the same functionality as `xhr` adapter, **including upload and download progress capturing**. 
+Also, it supports additional response types such as `stream` and `formdata` (if supported by the environment).
 
 ## Semver