Site logo (#284)

* Add the option to use a logo for the top left site title

README.md

 Some scripts need to be added within the page head. To add your own scripts to the page head, simply insert them into the `head-additions.html` partial located in the `layouts/partials` folder.
 
 
+### Logo
+
+You can replace the title of your site in the top left corner of each page with your own logo. To do that put your own logo into the `static` directory of your website, and add the `site_logo` parameter to the site params in your config file. For example:
+
+```
+[params]
+  site_logo = "img/logo.svg"
+```
+
+
 ### Nearly finished
 
 In order to see your site in action, run Hugo's built-in local server.

exampleSite/config.toml

 
 [params]
   favicon = ""
+  site_logo = ""
   description = "The last theme you'll ever need. Maybe."
   facebook = ""
   twitter = "https://twitter.com/GoHugoIO"

layouts/partials/site-navigation.html

 <nav class="pv3 ph3 ph4-ns" role="navigation">
   <div class="flex-l justify-between items-center center">
     <a href="{{ .Site.Home.RelPermalink }}" class="f3 fw2 hover-white no-underline white-90 dib">
-      {{ .Site.Title }}
+      {{ with .Site.Params.site_logo }}
+        <img src="{{ . }}" class="w100 mw5-ns" alt="{{ $.Site.Title }}" />
+      {{ else }}
+        {{ .Site.Title }}
+      {{ end }}
     </a>
     <div class="flex-l items-center">
       {{ partial "i18nlist.html" . }}