feat(doc): add README

README.md

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+# php-globalsnapshot
+
+Capture the current state of all [PHP superglobal variables](https://www.php.net/manual/en/language.variables.superglobals.php); which can then be restored to that state at a later time.
+
+Example use:
+```php
+// Initial state
+$_ENV["hello"] = "world"; // echo: "world"
+
+// Capture initial state
+$snapshot = (new GlobalSnapshot())->capture();
+
+// Manipulate superglobals
+$_ENV["hello"] .= " and mom!"; // echo: "world and mom"
+
+// Restore initial state
+$snapshot->restore();
+
+// Initial state restored
+echo $_ENV["hello"]; // echo: "world"
+```
+
+# Quickstart
+
+1. Install with composer
+```
+composer install victorwesterlund/globalsnapshot
+```
+
+2. `use` in your project
+```php
+use victorwesterlund\GlobalSnapshot;
+```
+
+3. Capture current superglobals with `capture()`
+```php
+// Initialize a new GlobalSnapshot instance to store current values (one snapshot per instance)
+$snapshot = new GlobalSnapshot();
+
+// Capture current superglobal state
+$snapshot->capture();
+```
+
+3. Restore superglobals with `restore()`
+```php
+// ... some other code
+
+// Restore superglobals to state of `capture()`
+$snapshot->restore();
+```