Add notes on benchmarking to README

Author: tsolomko

README.md

@@ -78,3 +78,18 @@ If you'd like to contribute code, please [create a pull request](https://github.
 
 __Note:__ If you are considering working on BitByteData, please note that Xcode project (BitByteData.xcodeproj)
 was created manually and you shouldn't use `swift package generate-xcodeproj` command.
+
+### Performance and benchmarks
+
+One of the most important goals of BitByteData's development is high speed performance. To help achieve this goal there
+are benchmarks for every function in the project as well as a handy command-line tool, `benchmarks.py`, which helps to
+run, show, and compare benchmarks and their results.
+
+If you are considering contributing to the project please make sure that:
+
+1. Every new function has also a new benchmark added.
+2. Every other change to any existing function doesn't introduce performance regressions, or, at the very least, these
+   regressions are small and such performance tradeoff is necessary and justifiable.
+
+Finally, please note that any meaningful comparison can be made only between benchmarks run on the same hardware and
+software system.