Add migration guide for removed null entropy config option

Signed-off-by: Mateusz Starzyk <[email protected]>

docs/3.0-migration-guide.d/remove-null-entropy.md

@@ -0,0 +1,9 @@
+Remove the option to build the library without any entropy sources
+------------------------------------------------------------------
+
+This does not affect users who use the default `config.h`, as this option was
+already off by default.
+
+If you were using the `MBEDTLS_TEST_NULL_ENTROPY` option, you can either use
+`MBEDTLS_ENTROPY_NV_SEED` or create a fake entropy function.
+