BaseTools/Pkcs7: Add readme.md
BaseTools/Pkcs7: Add readme.md

Add readme.md to describe the X.509 certificate generation.

Cc: Yonghong Zhu <78d981fe33a016071af5222c56d5cf77f30fb0b2@intel.com>
Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Cc: Michael D Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>
Cc: Qin Long <39a52309c812a080396e90c26873d3f69837761a@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <364a90bfebd1f362ebb7b48e4bf8ec010adef203@intel.com>
Reviewed-by: Qin Long <39a52309c812a080396e90c26873d3f69837761a@intel.com>

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