Add README.md

Author: aboks

README.md

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+:elephant: php-src-devtools :whale:
+===================================
+Tools to make it easier to contribute to PHP itself ([php-src](https://github.com/php/php-src)). Uses Docker containers for building, running and testing PHP from source.
+
+Installation
+------------
+Ensure that [Docker](https://www.docker.com/get-docker) is installed and running on your system. Clone or download this repository somewhere, and ensure that `php-src-devtools` is on your `PATH`.
+
+Usage
+-----
+In the folder that contains your clone of php-src, run:
+* `php-src-devtools build` to build PHP. Note that the initial build may take about an hour.
+* `php-src-devtools test` to run the full PHP testsuite. You can use `php-src-devtools test path/to/test.phpt` to run a single test, or `php-src-devtools test ext/foo` to test a single extension.
+* `php-src-devtools php` to run the compiled PHP CLI, e.g. `php-src-devtools php -i` for phpinfo.
+
+License
+-------
+The code is released under the MIT license.