Update documentation of runtime benchmarks directory

Author: Kobzol

collector/runtime-benchmarks/README.md

@@ -2,8 +2,23 @@
 This directory contains various pieces of code for which we measure how fast do they execute
 when they are compiled with a specific version of `rustc`.
 
-The benchmarks are located in crates that are part of the `runtime-benchmarks` workspace. Each crate
-contains a set of benchmarks defined using named closures.
+The benchmarks are located in several crates (`benchmark groups`) located in this directory. Each
+group defines a set of benchmarks using named closures. Each group should have a single,
+default, binary target, which will be executed by `collector`, and it should use the
+`run_benchmark_group` function from [`benchlib`](../benchlib) to define the benchmarks.
 
-Benchmarks are divided into sub-crates so that some benchmarks can use various versions of dependency
-crates and also so that they are grouped together by a relevant area (e.g. hashmap benchmarks).
+Runtime benchmarks are divided into groups so that some benchmarks can use different versions of
+dependency crates and also so that they are grouped together by a relevant area
+(e.g. hashmap benchmarks).
+
+## How are benchmarks executed
+The `collector` compiles each benchmark group and then invokes it with the `list` argument to list
+all benchmarks contained in the group.
+
+Then it executes each group with the `run` argument, which will cause `benchlib` to actually perform
+the benchmarks and output the results to `stdout` in JSON. That means that the benchmarked code should
+not print anything to `stdout`!
+
+Note that each benchmark group binary will be thus invoked twice per benchmarking collection. Keep this
+in mind so that the `main` function of the group doesn't perform too much work, which will be thrown
+away when it is invoked with the `list` argument.