Added basic info

README.md

@@ -1,2 +1,44 @@
 # QtPBFImagePlugin
-Qt image plugin for displaying Mapbox PBF tiles
+Qt image plugin for displaying Mapbox vector tiles
+
+## Description
+QtPBFImagePlugin is a Qt image plugin that enables applications capable of
+displaying raster MBTiles maps to also display vector (PBF) tiles without
+(almost, see usage) any application modifications.
+
+Standard Mapbox GL Styles are used for styling the maps. Most style features
+used by [Maputnik](http://editor.openmaptiles.org) are supported. The style
+is loaded from /usr/share/pbf/style.json or ~/.pbf/style.json if it exists.
+
+## Usage
+Due to a major design flaw in the Mapbox vector tiles specification (the zoom
+is not part of the PBF data), the plugin can not be used "as is", but passing
+the zoom level is necessary. This is done by exploiting the optional format
+parameter of the QImage constructor or the QImage::fromData() or
+QPixmap::loadFromData() functions. The zoom number is passed as ASCII string
+to the functions:
+```cpp
+QPixmap pm;
+pm.loadFromData(tileData(_zoom), QString::number(_zoom).toLatin1());
+```
+
+## Build
+Build requirements:
+* QT 5.x
+* Google Protocol Buffers (protobuf-lite)
+
+Build steps:
+```shell
+qmake gpxsee.pro
+make
+```
+
+## Install
+Copy the plugin to the system Qt image plugins path to make it work. You may
+also set the QT_PLUGIN_PATH system variable before starting the application.
+
+## Limitations
+As the plugin only has isolated info about a single tile image, texts
+overlapping to neighbour tiles can not be displayed or their overlapping can
+not be avoided (the plugin uses the first approach). This is a principal
+constraint that can't be evaded.