Create README.md

build/README.md

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+# /build directory
+
+The /build directory serves 2 purposes in the tree:
+
+1. some top-level makefiles and developer workflows use build/XXX as the configured build tree
+2. some parts of the CHIP build system live here
+
+For this reason, most of this directory is "ignore".
+
+If you update the contents of this directory, please also update the top level
+.gitignore file.