Make compatibility chart tabular

Author: martin-helmich

README.md

@@ -13,7 +13,15 @@ This library is [MIT-licensed](LICENSE.txt).
 
     $ composer require helmich/phpunit-psr7-assert
 
-**Compatibility notice**: [Version 1](https://github.com/martin-helmich/phpunit-psr7-assert/tree/v1) (the `v1` branch) of this library is compatible with PHPUnit 4.8 to 5. [Version 2](https://github.com/martin-helmich/phpunit-psr7-assert/tree/v2) (the `v2` branch) is compatible with PHPUnit 6. [Version 3](https://github.com/martin-helmich/phpunit-psr7-assert/tree/master) (the `master` branch) is compatible with PHPUnit 7. When using `composer require`, Composer should automatically pick the correct version for you.
+## Compatibility
+
+There are several release branches of this library, each of these being compatible with different releases of PHPUnit and PHP. The following table should give an easy overview:
+
+| PSR-7 assertion version | PHPUnit 4 | PHPUnit 5 | PHPUnit 6 | PHPUnit 7 |
+| ----------------------- | --------- | --------- | --------- | --------- |
+| v1 (branch `v1`), **unsupported** | :white_check_mark: | :white_check_mark: | :no_entry_sign: | :no_entry_sign: |
+| v2 (branch `v2`) | :no_entry_sign: | :no_entry_sign: | :white_check_mark: | :no_entry_sign: |
+| v3 (branch `master`) | :no_entry_sign: | :no_entry_sign: | :no_entry_sign: | :white_check_mark: |
 
 ## Usage