Documentation overhaul.

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

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

docs/cli_help_axis.md

@@ -0,0 +1,69 @@
+# Axis Specification
+
+The `--axis <axis spec>` option redefines the values in a benchmark's axis. It
+applies to the benchmark created by the most recent `--benchmark` argument, or
+all benchmarks if it precedes all `--benchmark` arguments (if any).
+
+Valid axis specification follow the form:
+
+* `<axis_name>=<value>`
+* `<axis_name>=[<value1>,<value2>,...]`
+* `<axis_name>=[<start>:<stop>]`
+* `<axis_name>=[<start>:<stop>:<stride>]`
+* `<axis_name>[<flags>]=<value>`
+* `<axis_name>[<flags>]=[<value1>,<value2>,...]`
+* `<axis_name>[<flags>]=[<start>:<stop>]`
+* `<axis_name>[<flags>]=[<start>:<stop>:<stride>]`
+
+Whitespace is ignored if the argument is quoted.
+
+The axis type is taken from the benchmark definition. Some axes have additional
+restrictions:
+
+* Numeric axes:
+  * A single value, explicit list of values, or strided range may be specified.
+  * For `int64` axes, the `power_of_two` flag is specified by adding `[pow2]`
+    after the axis name.
+  * Values may differ from those defined in the benchmark.
+* String axes:
+  * A single value or explicit list of values may be specified.
+  * Values may differ from those defined in the benchmark.
+* Type axes:
+  * A single value or explicit list of values may be specified.
+  * Values **MUST** be a subset of the types defined in the benchmark.
+  * Values **MUST** match the input strings provided by `--list` (e.g. `I32`
+    for `int`).
+  * Provide a `nvbench::type_strings<T>` specialization to modify a custom
+    type's input string.
+
+# Examples
+
+## Single Value
+
+| Axis Type | Example                 | Example Result   |
+|-----------|-------------------------|------------------|
+| Int64     | `-a InputSize=12345`    | 12345            |
+| Int64Pow2 | `-a InputSize[pow2]=8`  | 256              |
+| Float64   | `-a Quality=0.5`        | 0.5              |
+| String    | `-a RNG=Uniform`        | "Uniform"        |
+| Type      | `-a ValueType=I32`      | `int32_t`        |
+
+## Explicit List
+
+| Axis Type | Example                         | Example Result                 |
+|-----------|---------------------------------|--------------------------------|
+| Int64     | `-a InputSize=[1,2,3,4,5]`      | 1, 2, 3, 4, 5                  |
+| Int64Pow2 | `-a InputSize[pow2]=[4,6,8,10]` | 16, 64, 256, 1024              |
+| Float64   | `-a Quality=[0.5,0.75,1.0]`     | 0.5, 0.75, 1.0                 |
+| String    | `-a RNG=[Uniform,Gaussian]`     | "Uniform", "Gaussian"          |
+| Type      | `-a ValueType=[U8,I32,F64]`     | `uint8_t`, `int32_t`, `double` |
+
+## Strided Range
+
+| Axis Type | Example                         | Example Result               |
+|-----------|---------------------------------|------------------------------|
+| Int64     | `-a InputSize=[2:10:2]`         | 2, 4, 6, 8, 10               |
+| Int64Pow2 | `-a InputSize[pow2]=[2:10:2]`   | 4, 16, 64, 128, 256, 1024    |
+| Float64   | `-a Quality=[.5:1:.1]`          | 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 |
+| String    | [Not supported]                 |                              |
+| Type      | [Not supported]                 |                              |