README: mv logo.svg include/logo.{png,svg}

Some Git hoster (e.g. sr.ht) disable hosting of svg images (xml) to
avoid XSS attacks. To show the logo correctly on all code hosters use a
"safe" PNG image.

Also move logo(s) to include/ folder to lower autocomplete churn with
the `logs/` folder. While at it, replace absolute logo path and make it
relative, as this may break other code hosters as well.

Signed-off-by: Paul Spooren <mail@aparcar.org>

README.md

@@ -1,4 +1,4 @@
-![OpenWrt logo](/logo.svg)
+![OpenWrt logo](include/logo.png)
 
 OpenWrt Project is a Linux operating system targeting embedded devices. Instead
 of trying to create a single, static firmware, OpenWrt provides a fully