doc(README): fix missing link

Author: everythingfunctional

README.md

@@ -105,3 +105,4 @@ This library is released under a [BSD-3 license].
 [read-from-json.f90]:./example/read-from-json.f90
 [dag-to-dot.f90]: ./example/dag-to-dot.f90
 [documentation]: https://sourceryinstitute.github.io/dag/
+[issue]: https://github.com/sourceryinstitute/dag/issues/new