advotracker_client: introduce Windows icon handling

* include `windres` crate
* create a `build.rs`
* include a `hotline.svg` that is converted to advotracker.ico
  using inkscape and imagemagick
* crate a `README.md`

Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

crates/advotracker_client/README.md

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+# About
+
+This is the client component of `advotracker`.
+
+## Windows specific
+
+We like to support an icon an object attributes for the compiled
+version of `advotracker`.  While searching for a solution, the description of this
+[Web link][https://graphicdesign.stackexchange.com/questions/77359/how-to-convert-a-square-svg-to-all-size-ico]
+was adapted and combined with a `windres` crate.
+
+### Icon creation
+
+This commandline solution will create an icon that can be linked into the Windows binary.
+
+* create the `svg` art file
+  You may use `inkscape`, a versitile and user frendly
+  tool to create your vector graphics art. Save it as `advotracker.svg`. Next export the scaled png files.
+
+```
+inkscape -w 16 -h 16 -o advotracker_16.png -l advotracker.svg
+inkscape -w 32 -h 32 -o advotracker_32.png -l advotracker.svg
+inkscape -w 48 -h 48 -o advotracker_48.png -l advotracker.svg
+inkscape -w 64 -h 64 -o advotracker_64.png -l advotracker.svg
+```
+* create the `ico` file
+  The ico format can handle multiple layes with different resolutions.
+  We make use of ImageMagick to create the icon file like this:
+
+```
+convert advotracker_16.png advotracker_32.png advotracker_48.png advotracker_64.png advotracker.ico
+```
+
+* link in the icon
+
+rust offers the `windres` crate, that will handle the linking of icons
+as well as the definition of binary objet resources. `advotracker`
+will define the needed settings in its `build.rs` file.