Specify Composition Chapter Structure

spec/Section 3 -- Composite Schema.md

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+# Composite Schema
+
+The supergraph API schema is a subset of the supergraph schema that includes
+just those types and fields that should be accessible by clients. Metadata used
+by the router is excluded from the API schema by default. In addition, subgraphs
+can mark definitions as @inaccessible to exclude them from the API schema. This
+ensures these fields can only be used by the router (to access a key field and
+use it as input to another subgraph for example) or by other subgraphs (by
+requesting it in a @requires).