Hello 3.x development branch

Once this PR is merged, we can start working on the new [v3.0.0 milestone](https://github.com/reactphp/stream/milestone/22). The default branch will be `3.x` and the old `1.x` branch still stay in place at least until `3.0.0` is released.

Refs:
Road map ticket for stream: reactphp#173
Plans for ReactPHP v3: https://github.com/orgs/reactphp/discussions/481
PR templated from: friends-of-reactphp/mysql#185

README.md

@@ -5,6 +5,14 @@
 
 Event-driven readable and writable streams for non-blocking I/O in [ReactPHP](https://reactphp.org/).
 
+> **Development version:** This branch contains the code for the upcoming v3
+> release. For the code of the current stable v1 release, check out the
+> [`1.x` branch](https://github.com/reactphp/stream/tree/1.x).
+>
+> The upcoming v3 release will be the way forward for this package. However,
+> we will still actively support v1 for those not yet on the latest version.
+> See also [installation instructions](#install) for more details.
+
 In order to make the [EventLoop](https://github.com/reactphp/event-loop)
 easier to use, this component introduces the powerful concept of "streams".
 Streams allow you to efficiently process huge amounts of data (such as a multi
@@ -1199,11 +1207,11 @@ $source->pipe($dest);
 The recommended way to install this library is [through Composer](https://getcomposer.org).
 [New to Composer?](https://getcomposer.org/doc/00-intro.md)
 
-This project follows [SemVer](https://semver.org/).
-This will install the latest supported version:
+Once released, this project will follow [SemVer](https://semver.org/).
+At the moment, this will install the latest development version:
 
 ```bash
-composer require react/stream:^1.3
+composer require react/stream:^3@dev
 ```
 
 See also the [CHANGELOG](CHANGELOG.md) for details about version upgrades.