Add readme

README.md

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+# Usage:
+```
+docker build -t backupinator .
+
+docker run \
+    -v /path/to/folder1:/in/folder1:ro \
+    -v /path/to/folder2:/in/folder2:ro \
+    -v /path/to/output:/output \
+    -v /path/to/key1.pub:/keys/key1.pub:ro \
+    -v /path/to/key2.pub:/keys/key2.pub:ro \
+    backupinator
+```
+