Move perf-runner.md to the docs/ directory.

Because when I looked for it yesterday I failed to find it, and `docs/`
was one of the places I did look.

Also link to the glossary from the collector README.

Author: nnethercote

collector/README.md

@@ -1,7 +1,8 @@
 # Rust Compiler Performance Benchmarking and Profiling
 
 Hardware and software details of the machine that executes the CI details can be found
-[here](perf-runner.md). 
+[here](../docs/perf-runner.md). A glossary of relevant terms can be found
+[here](../docs/glossary.md).
 
 ## The benchmarks