Create .gitkeep file in doc/en to make build pass

If doc/en does not exist you'll see something like this when running make:

```
...
Making all in doc
make[1]: Entering directory `/opt/soup-sharp/doc'
Generating soup-sharp-docs.source
/usr/bin/monodocer -assembly:../out/soup-sharp.dll -path:en
mdoc: Directory '/opt/soup-sharp/doc/en' not found.
See `mdoc help' for more information.
make[1]: *** [en/*/*.xml] Fehler 1
make[1]: Leaving directory `/opt/soup-sharp/doc'
make: *** [all-recursive] Fehler 1
```