Add missing pear lib

```pecl install vips

Command 'pecl' not found, but can be installed with:

apt install php-pear
```

after adding pear all works fine

Author: mstaack

README.md

@@ -32,7 +32,7 @@ See the README there, but briefly:
    binaries on the vips website. For example, on Debian:
 
    ```
-   sudo apt-get install libvips-dev
+   sudo apt-get install libvips-dev php-pear
    ```
 
    Or macOS: