Add fork info.

README.md

+**This is fork of the original [bigdataviewer/bigdataviewer-core](https://github.com/bigdataviewer/bigdataviewer-core) repository.**
+
 # bigdataviewer-core
-[![](https://api.travis-ci.org/bigdataviewer/bigdataviewer-core.svg?branch=master)](https://travis-ci.org/bigdataviewer/bigdataviewer-core)
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 ImgLib2-based viewer for registered SPIM stacks and more