Update docs.

cmd/pembody/README

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+pembody
+
+This is a utility for decoding PEM-encoded files and spewing their
+contents to standard output. No trailing newline is appended.
+
+Example:
+        $ cat file.pem
+        -----BEGIN TEST DATA-----
+        c2FtcGxlIGJpbmFyeSBkYXRh
+        -----END TEST DATA-----
+        $ pembody file.pem
+        sample binary data$
+