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 <csmartdalton@google.com>
Commit-Queue: Nathaniel Nifong <nifong@google.com>

tools/skpbench/README.md

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+# skpbench
+
+skpbench is a benchmarking tool for replaying skp or mksp files on android devices.
+it achieves a lower variance in framerate by controlling the clock speed and stopping
+all other processes that could cause interference.
+
+## Build
+
+skpbench consists of the skpbench binary which must be built for the phone you intend to run on,
+and skpbench.py which runs on the machine the phone is connected to via ADB and is the entry point.
+
+The to build skia for android are at https://skia.org/user/build#android and reproduced here.
+
+Download the Android NDK
+
+```
+cipd auth-login
+python2 infra/bots/assets/android_ndk_linux/download.py  -t /tmp/ndk
+
+```
+
+After this is set up once, build skpbench for your target cpu (assumed to be arm64 here for a Pixel 3)
+
+```
+bin/gn gen out/arm64 --args='ndk="/tmp/ndk" target_cpu="arm64" is_debug=false'
+ninja -C out/arm64 skpbench
+```
+
+## Benchmark an SKP on a connected device.
+
+First, copy the built skpbench binary and an example skp file to the device.
+
+```
+adb push out/arm64/skpbench /data/local/tmp
+adb push /home/nifong/Downloads/foo.skp /data/local/tmp/skps/
+```
+
+Run skpbench.py (in my case on a Pixel 3)
+
+```
+python tools/skpbench/skpbench.py \
+  --adb \
+  --force \
+  --config gles \
+  /data/local/tmp/skpbench \
+  /data/local/tmp/skps/foo.skp
+```
+
+`--adb` specifies that it should use adb to the only connected device and run skpbench there.
+`--force` is necessary because we don't yet have a configuration to monitor vitals on the Pixel 3.
+`--config gles` specifies Open GL ES is the backend GPU config to use.
+
+Additional documentation of arguments is printed by `python tools/skpbench/skpbench.py --help`
+
+Output appears in the following format
+```
+   accum    median       max       min   stddev  samples  sample_ms  clock  metric  config    bench
+  0.1834    0.1832    0.1897    0.1707    1.59%      101         50  cpu    ms      gles      foo.skp
+```
+
+`accum` is the time taken to draw all frames, divided by the number of frames.
+`metric` specifies that the unit is ms (milliseconds per frame)
+
+## Production
+
+skpbench is run as a tryjob from gerrit, where it uploads the results to perf.skia.org.
+TODO(nifong, csmartdalton): elaborate on this section.