Add a readme

README.md

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+# shim-build
+
+Build [shim](https://github.com/rhboot/shim) in a Docker container.
+
+## Makefile targets
+
+Build shim in a Docker container:
+
+    build
+    
+Build with the cache turned off to get the full build log:
+    
+    build-no-cache
+    
+Copy the shim builds from the container to the host:
+    
+    copy