Fix markdownlint warnings, and highlight release-branch distinction.

README.md

@@ -19,8 +19,17 @@ thread for Perfetto. The binary size is typically about 450KB.
 Clone a [recent release](https://github.com/google/perfetto/releases) of
 Perfetto to get the Perfetto SDK (perfetto.h and perfetto.cc).
 
-Configure and build PerCetto:
+e.g.
+
+```sh
+git clone https://github.com/google/perfetto -b release/v23.x
 ```
+
+(Only the release branches contain the amalgamated SDK source files.)
+
+Configure and build PerCetto:
+
+```sh
 meson build -Dperfetto-sdk=path/to/perfetto/sdk
 meson compile -C build
 ```