Convert platforms to table

Author: dtolnay

README.md

@@ -54,19 +54,16 @@ $ buck2 build :rustc_ast[profile][llvm_passes] --show-output
 
 The following platforms are available for use with `--target-platforms`:
 
-- `//platforms/stage1:compiler`
-- `//platforms/stage1:compiler-build-script`
-- `//platforms/stage1:library`
-- `//platforms/stage1:library-build-script`
-- `//platforms/stage2:compiler`
-- `//platforms/stage2:compiler-build-script`
-- `//platforms/stage2:library`
-- `//platforms/stage2:library-build-script`
-
-The "stage1 compiler" platforms compile Rust code using stage0 (downloaded)
-rustc and rustdoc and clippy. "Stage1 library" uses stage1 (built from source)
-tools. "Stage2 compiler" uses the stage1 tools and stage1 standard library, and
-"stage2 library" uses stage2 tools.
+| target platform | uses rustc, rustdoc, clippy | uses standard library |
+|---|---|---|
+| `//platforms/stage1:compiler` | downloaded stage0 | downloaded stage0 |
+| `//platforms/stage1:compiler-build-script` | downloaded stage0 | downloaded stage0 |
+| `//platforms/stage1:library` | built-from-source stage1 | none |
+| `//platforms/stage1:library-build-script` | downloaded stage0 | downloaded stage0 |
+| `//platforms/stage2:compiler` | built-from-source stage1 | built-from-source stage1 |
+| `//platforms/stage2:compiler-build-script` | built-from-source stage1 | built-from-source stage1 |
+| `//platforms/stage2:library` | built-from-source stage2 | none |
+| `//platforms/stage2:library-build-script` | built-from-source stage1 | built-from-source stage1 |
 
 The "build-script" platforms compile without optimization. These are used for
 procedural macros and build.rs. The non-build-script platforms compile with a