Added instruction for existing ZLIB configuration

[abdul-sami]

Using two configuration flags DZLIB_INCLUDE_DIR and DZLIB_LIB we can locate an existing ZLIB on the system

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

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[abdul-sami]

I signed it!

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