Expose flag for critical extensions

Enables creating X.509 CSRs with critical extensions.

Signed-off-by: Christoph Reiter <christoph.reiter@infineon.com>
diff --git a/docs/3.0-migration-guide.d/csr-add-critical-extension.md b/docs/3.0-migration-guide.d/csr-add-critical-extension.md
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index 0000000..ebcb343
--- /dev/null
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@@ -0,0 +1,9 @@
+Change the API to allow adding critical extensions to CSRs
+------------------------------------------------------------------
+
+This affects applications that call the `mbedtls_x509write_csr_set_extension`
+function.
+
+The API is changed to include the parameter `critical` which allow to mark an
+extension included in a CSR as critical. To get the previous behaviour pass
+`0`.