Update Readme

README.md

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 # php-extension-check
+Check your PHP project's PHP extension requirements against all loaded PHP extensions.
 
+Requires a **PHP version** of **7.2** or above.
+
+The idea behind this project is to have an idea of your project's requirements for a minimal server installation.
+
+Another use case can be cleaning up old legacy projects using the same server configuration/installation for decades after doing some refactorings or upgrades, esp. when moving those projects into Docker images.
+
+## Installation
+*... todo, when any releases are ready*
+
+## Usage
+To check your code, simply run the `extension-check.phar`.
+
+The tool automatically executes the checks for the current directory (`./`).
+
+### Important
+Please note that your command line and web server PHP configurations (php.ini files) may load different PHP extensions!
+
+To use a specific `php.ini` file, run the tool like this:
+```php
+php -c path/to/your/webserver/php.ini ./extension-check.phar
+```
+
+### Checking a specific Directory
+If you want to check a specific directory, you can specify it as an argument:
+```php
+./extension-check.phar ./src
+```
+Note: You should include your vendor files, if any, for valid results.
+
+### Exclude Extensions
+If you want to exclude some PHP extensions, you can use the `--exclude` option:
+```php
+./extension-check.phar --exclude=core,standard,xdebug
+```
+or alternatively
+```php
+./extension-check.phar --exclude=core --exclude=standard --exclude=xdebug
+```
+
+## Contributions
+To check the code quality, run
+```php
+composer check
+```
+
+To run the test suite, execute
+```php
+composer test
+```