Add description of tram type indicator

 index.css

@@ -39,6 +39,9 @@
    margin: -5px 0;
    font-size: 21px;
}
.bigger {
    font-size: 21px;
}

nav.loading {
    background-image: -webkit-linear-gradient(45deg,rgba(0,0,0,.1) 25%,transparent 25%,transparent 50%,rgba(0,0,0,.1) 50%,rgba(0,0,0,.1) 75%,transparent 75%,transparent);

 index.html

@@ -97,6 +97,10 @@
                        <span class="label bg-danger">Delayed 10′+</span>
                    </p>
                    
                    <p class="small">
                        <span class="bigger" title="Bombardier NGT6, Bombardier NGT8, PESA 2014N Krakowiak, Newag Nevelo 126N">&#x267F;</span> - low-floor tram. <span class="bigger" title="Düwag/Man/MPK N8C-NF, Düwag/MPK GT8C, Bombardier-Rotax/MPK EU8N, Protram 405N-Kr">*&#x267F;</span> - partially low-floor tram. <span class="bigger" title="other tram types">&#x2010;</span> - high-floor tram. Hover the icon for more details.
                    </p>
					
                    <p class="small">Relative times (eg. 3 min) are real-time arrivals based on tram location data. Absolute times (eg. 8:01) are scheduled departures, shown when the tram cannot be located. "?" means unknown delay.</p>
                    
                    <p class="small">&copy; 2016 Jacek Kowalski - <a href="https://github.com/jacekkow/mpk-ttss">Source</a> - <a href="https://raw.githubusercontent.com/jacekkow/mpk-ttss/master/LICENSE">License</a></p>