Document -Z crate-attr

Author: jyn514

Diff for: src/doc/unstable-book/src/compiler-flags/crate-attr.md

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+# `crate-attr`
+
+The tracking issue for this feature is: [#138287](https://github.com/rust-lang/rust/issues/138287).
+
+------------------------
+
+The `-Z crate-attr` flag allows you to inject attributes into the crate root.
+For example, `-Z crate-attr=crate_name="test"` acts as if `#![crate_name="test"]` were present before the first source line of the crate root.