readme: document Trace and Error Reporting clients
readme: document Trace and Error Reporting clients

Fixes #656.

Change-Id: I8344ac86ca3d46ec0b6fb264efa76b8100c6958f
Reviewed-on: https://code-review.googlesource.com/14050
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go