Add README

Author: martin4861

test/small_world/.bazelrc

@@ -1,10 +1,8 @@
 common --@rules_swiftnav//cc:enable_bzlmod=true
 
-# # build:use_llvm20 --platforms=@rules_swiftnav//platforms:x86_64_darwin_llvm20
-# # build:use_llvm20 --platforms=@rules_swiftnav//platforms:aarch64_darwin_llvm20
-# build:use_llvm20 --platforms=@rules_swiftnav//platforms:x86_64_linux_llvm20
-# # build:use_llvm20 --platforms=@rules_swiftnav//platforms:aarch64_linux_llvm20
 
+coverage --experimental_generate_llvm_lcov
+coverage --combined_report=lcov
 build:coverage_x86_64_linux --platforms=@rules_swiftnav//platforms:x86_64_linux_llvm20
 
 build:coverage_aarch64_darwin --platforms=@rules_swiftnav//platforms:aarch64_darwin_llvm20

test/small_world/README.md

@@ -0,0 +1,13 @@
+# Coverage
+
+on `x86_64 ubuntu`:
+
+```
+bazel coverage --config=coverage_x86_64_linux //src/base_math:base_math_test
+
+cp "$(bazel info output_path)/_coverage/_coverage_report.dat" cov.dat
+
+genhtml --output html cov.dat
+```
+
+Mac is not yet supported.