Document supported-crate-types print request in unstable book

Author: jieyouxu

Diff for: src/doc/unstable-book/src/compiler-flags/print-supported-crate-types.md

@@ -0,0 +1,27 @@
+# `print=supported-crate-types`
+
+The tracking issue for this feature is: [#138640](https://github.com/rust-lang/rust/issues/138640).
+
+------------------------
+
+This option of the `--print` flag produces a list of crate types (delimited by newlines) supported for the given target.
+
+The crate type strings correspond to the values accepted by the `--crate-type` flag.
+
+Intended to be used like this:
+
+```bash
+rustc --print=supported-crate-types -Zunstable-options --target=x86_64-unknown-linux-gnu
+```
+
+Example output for `x86_64-unknown-linux-gnu`:
+
+```text
+bin
+cdylib
+dylib
+lib
+proc-macro
+rlib
+staticlib
+```