Update changelog

Author: mdboom

CHANGELOG.md

@@ -2,12 +2,63 @@
 
 ## v2.0.0
 
-Most of the work has moved from GitHub Actions `.yml` files to Python code in `workflow.py`.
-In the future, this will allow supporting more workflow engines beyond just GitHub Actions.
+### Moving more code to Python
 
-**Migration note**: After running `python -m bench_runner install` to update your local files, but sure to add the new `workflow_bootstrap.py` file to your git repository.
+Most of the code to orchestrate the benchmarks at a high level has moved from
+GitHub Actions `.yml` files to Python code in `workflow.py`. In the future, this
+will allow supporting more workflow engines beyond just GitHub Actions.
 
-### New configuration
+**Migration note**: After running `python -m bench_runner install` to update
+your local files, be sure to add the new `workflow_bootstrap.py` file to your
+git repository.
+
+### Decoupling compiler configuration from tail-calling
+
+Previously, bench_runner had a flag, `CLANG`, that both (a) built with clang 19
+or later and (b) built the tail calling interpreter. This has been replaced with
+a new flag `TAILCALL` that only sets the flags to build with the tail calling
+interpreter. It is now up to the user to select a machine configured with a
+compiler that supports the tail calling interpreter.
+
+For machines with a clang-19 or later installed these example machine
+configurations could be used in your `bench_runner.toml`. The exact details will
+depend on your distribution and method of installing clang, etc.:
+
+```toml
+[runners.linux_clang]
+os = "linux"
+arch = "x86_64"
+hostname = "pyperf"
+github_runner_name = "linux-x86_64-linux"
+include_in_all = false
+[runners.linux_clang.env]
+CC = "$(which clang-19)"
+LLVM_AR = "$(which llvm-ar-19)"
+LLVM_PROFDATA = "$(which llvm-profdata-19)"
+
+[runners.darwin_clang19]
+os = "darwin"
+arch = "arm64"
+hostname = "CPythons-Mac-mini.local"
+github_runner_name = "darwin-arm64-darwin"
+[runners.darwin_clang19.env]
+PATH = "$(brew --prefix llvm)/bin:$PATH"
+CC = "$(brew --prefix llvm)/bin/clang"
+LDFLAGS = "-L$(brew --prefix llvm)/lib"
+CFLAGS = "-L$(brew --prefix llvm)/include"
+
+[runners.pythonperf1_clang]
+os = "windows"
+arch = "x86_64"
+hostname = "WIN-5FKPU9U7KDT"
+github_runner_name = "windows-x86_64-pythonperf1"
+include_in_all = false
+[runners.pythonperf1_clang.env]
+BUILD_DEST = "PCBuild/amd64"
+PYTHON_CONFIGURE_FLAGS = '`"/p:PlatformToolset=clangcl`" `"/p:LLVMInstallDir=C:\Program Files\LLVM`" `"/p:LLVMToolsVersion=19.1.6`"'
+```
+
+### Controlling number of cores used for builds
 
 Runners have a new configuration `use_cores` to control the number of CPU cores
 used to build CPython. By default, this will use all available cores, but some