Add readme and gitignore

Author: lkdvos

.gitignore

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 experimental
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+/benchmark/results/

benchmark/README.md

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+# TensorKit Benchmarks
+
+This directory contains a benchmark suite for TensorKit.
+Most of the benchmarks are designed to capture performance characteristics of the library, and are not intended to be used as a benchmark suite for comparing different libraries.
+In particular, the main goal here is to catch performance regressions and/or improvements between different versions of TensorKit.
+
+## Running the benchmarks
+
+The benchmarks are written using `BenchmarkTools.jl`, and the full suite can be found in the `SUITE` global variable defined in `benchmarks.jl`.
+Sometimes, it is useful to run only a subset of the benchmarks.
+To do this, you can use the `--modules` flag to specify which modules to run.
+Alternatively, you can use the `TensorKitBenchmarks` module directly, which is designed after `BaseBenchmarks` to allow for conditional loading of the benchmarks.
+
+For a more streamlined CLI experience, you can use [`AirspeedVelocity.jl`](https://github.com/MilesCranmer/AirspeedVelocity.jl) to run the benchmarks.
+The following command will run the benchmarks and compare with the current master branch:
+
+```bash
+benchpkg TensorKit \
+    --rev=dirty,master \
+    -o benchmark/results/ \
+    -exeflags="--threads=4"
+```
+
+To compare with previous results, the following command can be used:
+
+```bash
+benchpkgtable TensorKit \
+    --rev=dirty,master \
+    -i benchmark/results/ \
+    -o benchmark/results/ \
+```