binman: Create README.entries

Create a new README containing documentation for the entry types supported
by binman. This provides an easy reference in one place. It is
automatically

binman: Create README.entries

Create a new README containing documentation for the entry types supported
by binman. This provides an easy reference in one place. It is
automatically generated from the source-code documentation.

Add a reference to this from the binman README.

Signed-off-by: Simon Glass <sjg@chromium.org>

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