Documentation overhaul.

Revamp README, split into multiple files. Add docs on CLI.

Add `--help` and `--help-axis`.

docs/cli_help.md

@@ -0,0 +1,93 @@
+# Queries
+
+* `--list`, `-l`
+  * List all devices and benchmarks without running them.
+
+* `--help`, `-h`
+  * Print usage information and exit.
+
+* `--help-axes`, `--help-axis`
+  * Print axis specification documentation and exit.
+
+# Output
+
+* `--csv <filename/stream>`
+  * Write CSV output to a file, or "stdout" / "stderr".
+
+* `--markdown <filename/stream>`, `--md <filename/stream>`
+  * Write markdown output to a file, or "stdout" / "stderr".
+  * Markdown is written to "stdout" by default.
+
+* `--quiet`, `-q`
+  * Suppress output.
+
+* `--color`
+  * Use color in output (markdown + stdout only).
+
+# Benchmark / Axis Specification
+
+* `--benchmark <benchmark name/index>`, `-b <benchmark name/index>`
+  * Execute a specific benchmark.
+  * Argument is a benchmark name or index, taken from `--list`.
+  * If not specified, all benchmarks will run.
+  * `--benchmark` may be specified multiple times to run several benchmarks.
+  * The same benchmark may be specified multiple times with different
+    configurations.
+
+* `--axis <axis specification>`, `-a <axis specification>`
+  * Override an axis specification.
+  * See `--help-axis`
+    for [details on axis specifications](./cli_help_axis.md).
+  * Applies to the most recent `--benchmark`, or all benchmarks if specified
+    before any `--benchmark` arguments.
+
+# Benchmark Properties
+
+* `--devices <device ids>`, `--device <device ids>`, `-d <device ids>`
+  * Limit execution to one or more devices.
+  * `<device ids>` is a single id, or a comma separated list.
+  * Device ids can be obtained from `--list`.
+  * Applies to the most recent `--benchmark`, or all benchmarks if specified
+    before any `--benchmark` arguments.
+
+* `--min-samples <count>`
+  * Gather at least `<count>` samples per measurement.
+  * Default is 10 samples.
+  * Applies to the most recent `--benchmark`, or all benchmarks if specified
+    before any `--benchmark` arguments.
+
+* `--min-time <seconds>`
+  * Accumulate at least `<seconds>` of execution time per measurement.
+  * Default is 0.5 seconds.
+  * If both GPU and CPU times are gathered, this applies to GPU time only.
+  * Applies to the most recent `--benchmark`, or all benchmarks if specified
+    before any `--benchmark` arguments.
+
+* `--max-noise <value>`
+  * Gather samples until the error in the measurement drops below `<value>`.
+  * Noise is computed as the percent relative standard deviation.
+  * Default is 0.5%.
+  * Only applies to Cold measurements.
+  * If both GPU and CPU times are gathered, this applies to GPU noise only.
+  * Applies to the most recent `--benchmark`, or all benchmarks if specified
+    before any `--benchmark` arguments.
+
+* `--skip-time <seconds>`
+  * Skip a measurement when a warmup run executes in less than `<seconds>`.
+  * Default is -1 seconds (disabled).
+  * Intended for testing / debugging only.
+  * Very fast kernels (<5us) often require an extremely large number of samples
+    to converge `max-noise`. This option allows them to be skipped to save time
+    during testing.
+  * Applies to the most recent `--benchmark`, or all benchmarks if specified
+    before any `--benchmark` arguments.
+
+* `--timeout <seconds>`
+  * Measurements will timeout after `<seconds>` have elapsed.
+  * Default is 15 seconds.
+  * `<seconds>` is walltime, not accumulated sample time.
+  * If a measurement times out, the default markdown log will print a warning to
+    report any outstanding termination criteria (min samples, min time, max
+    noise).
+  * Applies to the most recent `--benchmark`, or all benchmarks if specified
+    before any `--benchmark` arguments.