docs(README): add note about deprecation (denoland#3807)

README.md

@@ -25,7 +25,7 @@ Check out the documentation [here](https://deno.land/std?doc).
 
  Good:
  ```ts
- import { copy } from "https://deno.land/std@0.204.0/fs/copy.ts";
+ import { copy } from "https://deno.land/std@$STD_VERSION/fs/copy.ts";
  ```
 
 1. Only import modules that you require.
@@ -109,6 +109,24 @@ Check out the documentation [here](https://deno.land/std?doc).
 > For background and discussions regarding the stability of the following
 > sub-modules, see [#3489](https://github.com/denoland/deno_std/issues/3489).
 
+## Deprecation Policy
+
+We deprecate the APIs in the Standard Library when they get covered by new
+JavaScript language APIs or new Web Standard APIs. These APIs are usually
+removed after 3 minor versions.
+
+If you still need to use such APIs after the removal for some reason (for
+example, the usage in Fresh island), please use the URL pinned to the version
+where they are still available.
+
+For example, if you want to keep using `readableStreamFromIterable`, which was
+deprecated and removed in favor of `ReadableStream.from` in `v0.195.0`, please
+use the import URL pinned to `v0.194.0`:
+
+```ts
+import { readableStreamFromIterable } from "https://deno.land/[email protected]/streams/readable_stream_from_iterable.ts";
+```
+
 ## Contributing
 
 Check out the contributing guidelines [here](.github/CONTRIBUTING.md).