Add readme to benchmarks/ with some simple examples

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+
+# Benchmarks of Catch2
+
+This folder holds benchmarks for Catch2. It should not be built for the
+small(er) test sets, as it is only relevant for running (mainly runtime)
+benchmarks.
+
+Below you will find some practical examples using 
+[hyperfine](https://github.com/sharkdp/hyperfine) to determine the
+performance of various scenarios. They assume two parallel checkouts, one
+named `Catch2-old` and the other `Catch2-new`. You will need to change
+the paths to work on your own machine.
+
+
+## Runtime benchmarks
+
+The runtime benchmarks currently consist of few different `TEST_CASE`s,
+each with a simple loop over different assertion type. They are compiled
+into two binaries, one for assertion slow path and one for assertion fast
+path.
+
+
+### Assumptions and notes about real-world usage
+
+The basic assertion macro, `REQUIRE` (and `CHECK`) is the most common one,
+by far. Thus, it is the most important one to run quickly. It is probably
+followed by the matcher macro, `REQUIRE_THAT`.
+
+The most common result of an assertion is that it passes. Even if it is
+stringified and reported, that is most likely due to some listener/reporter
+that wants the string representation, not because it failed.
+
+The performance of both Debug and Release builds are important; users
+will run tests in both. LTO runtime performance is not too important,
+but compile time perf is.
+
+
+### Examples
+
+**Compare performance of `REQUIRE` in slow path, debug build**
+```text
+hyperfine --warmup 2 --shell none --parameter-list version old,new '/home/xarn/benches/Catch2-{version}/build-debug/benchmarks/AssertionsSlowPath -o /dev/null "REQUIRE"'
+```
+
+**Compare performance of `REQUIRE_THAT` in fast path, release build**
+```text
+hyperfine --warmup 2 --shell none --parameter-list version old,new '/home/xarn/benches/Catch2-{version}/build-release/benchmarks/AssertionsFastPath -o /dev/null "REQUIRE_THAT"'
+```
+
+**Compare performance of `REQUIRE` with stringification enabled, release build**
+```
+hyperfine --warmup 2 --shell none --parameter-list version old,new '/home/xarn/benches/Catch2-{version}/build-release/benchmarks/AssertionsFastPath -s -o /dev/null "REQUIRE"'
+```
+_Note that we redirect the output to `/dev/null` to reduce the overhead of the actual output printing, to see just the impact of stringification._
+
+
+## Compilation benchmarks
+
+As tests are often iterated upon and relinked, the compilation cost of
+Catch2 is also important.
+
+TODO:
+  * Single file vs test suite build
+  * Practical examples
+  * Include-only overhead
+  * Link-only example