Add client certificate recommendation document (#938)

Author: starwarfan

doc/certificates.md

@@ -0,0 +1,6 @@
+# Client side certificate management
+
+We recommand to use default credential storage to verify and manage certificates on different platforms.
+Windows: (https://docs.microsoft.com/en-us/windows-hardware/drivers/install/certificate-stores)
+IOS: (https://developer.apple.com/documentation/security/certificate_key_and_trust_services/certificates)
+Android: (https://developer.android.com/training/articles/security-config)