Add installation documentation

 INSTALL.md

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# Installation

## Requirements

Webserver with PHP 5.4+ (PHP 7+ recommended) and modules:
- iconv,
- PDO,
- PDO SQLite.

## Procedure

Just clone the repository into the top-level website directory:

```sh
cd /var/www
git clone https://git.jacekk.net/r/projects/mpk-ttss
```