[WIP] Adding a quick note explaining paramters.yml.dist a little

Hi guys!

This is really simple, but a WIP because I also want to add an additional link at the top to symfony/symfony-docs#4174 after it's created.

Thanks!

Maybe I'm wrong, but I think that there is a typo:

```
# This file is a "template" or what your ...

# This file is a "template" of what your ...
```