CryptoPkg: Add ECC related usage reference

CryptoPkg: Add ECC related usage reference

Describes the use cases under which ECC needs to be enabled,
and provides the impact on memory size for developers' reference.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Xiaoyu Lu <xiaoyux.lu@intel.com>
Cc: Guomin Jiang <guomin.jiang@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Yi Li <yi1.li@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>