Improve developer experience for eclipse users

[uweschaefer]

grpc-api does only build on cmdline because of os detection not configured in eclipse.

[mweirauch]

Thx. FYI, the 'os-maven-plugin' is also an Eclipse plugin. I usually dropped it into the Eclipse plugins folder and this did also do the trick. But this is of course better.

[uweschaefer]

jup. i found out just before and was about to add that to the readme, but this way it just works(tm) :D