Add docs/use-psa-crypto.md
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
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+This document describes the compile-time configutation option
+`MBEDTLS_USE_PSA_CRYPTO`: its current effects as well as some design
+considerations and plans for the future.
+
+Current effects
+===============
+
+(To be written.)
+
+Parts that are not affected yet
+===============================
+
+(To be written.)
+
+Design considerations
+=====================
+
+(To be written.)
+