Add Symfony Deprecation Contracts package

- Created CHANGELOG.md to maintain version history.
- Added README.md with usage instructions for the trigger_deprecation() function.
- Initialized composer.json for the Symfony Deprecation Contracts library, specifying dependencies and autoloading.

modules/pshowsso/deps/psr/http-client/README.md

@@ -0,0 +1,12 @@
+HTTP Client
+===========
+
+This repository holds all the common code related to [PSR-18 (HTTP Client)][psr-url].
+
+Note that this is not a HTTP Client implementation of its own. It is merely abstractions that describe the components of a HTTP Client.
+
+The installable [package][package-url] and [implementations][implementation-url] are listed on Packagist.
+
+[psr-url]: https://www.php-fig.org/psr/psr-18
+[package-url]: https://packagist.org/packages/psr/http-client
+[implementation-url]: https://packagist.org/providers/psr/http-client-implementation