Add a basic readme to help the uninitiated build and run skpbench
Add a basic readme to help the uninitiated build and run skpbench

Change-Id: Ib276f960f385c88392bca90924a9a42752e585ad
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295603
Reviewed-by: Chris Dalton <ef7fc3a08ada7d31f16e9a18b5f3c728256e041e@google.com>
Commit-Queue: Nathaniel Nifong <1ce561679465ea4956c6210637580cd7ba456816@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia