add licence

README.md

@@ -3,6 +3,7 @@
 [![build status](https://jenkins.ohsome.org/buildStatus/icon?job=oshdb-extract-generator/master)](https://jenkins.ohsome.org/blue/organizations/jenkins/oshdb-extract-generator/activity/?branch=master)
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 [![status: experimental](https://github.com/GIScience/badges/raw/master/status/experimental.svg)](https://github.com/GIScience/badges#experimental)
+[![LICENSE](https://img.shields.io/badge/license-GPL--v3-orange)](COPYING)
 
 Creates an extract in form of a OSHDB H2-file from an Ignite OSHDB database source.