Recommend adding target spec first, then std support later

Author: madsmtm

src/doc/rustc/src/target-tier-policy.md

@@ -125,6 +125,12 @@ To propose addition of a new target, open a pull request on [`rust-lang/rust`]:
 See also the documentation in the `rustc-dev-guide` on [adding a new target to
 `rustc`][rustc_dev_guide_add_target].
 
+Note that crates like `libc` and `cc` may not accept your patches before the
+`rustc` target specification itself exists, so it is recommended that you
+start out with a minimal pull request adding just that (i.e. only
+`#![no_core]` support), and then slowly expand the support in later pull
+requests.
+
 [tier3example]: https://github.com./rust-lang/rust/pull/94872
 [platform_template]: https://github.com./rust-lang/rust/blob/master/src/doc/rustc/src/platform-support/TEMPLATE.md
 [summary]: https://github.com./rust-lang/rust/blob/master/src/doc/rustc/src/SUMMARY.md