3af9258025 embedded: Document how to clean up linker flags (Tobin C. Harding)
Pull request description:
Our embedded crate includes instructions to source a shell script that sets the `RUSTFLAGS` env var. Having the env var set like this in ones environment breaks linkage when trying to do "regular" builds.
Document how to clean up.
ACKs for top commit:
apoelstra:
ACK 3af9258025
Kixunil:
ACK 3af9258025
Tree-SHA512: 72758fba4dede873da299f01d75fd64b549fe21f954c2720ba3b7fc3c29fd4ed28fd0a749bbe987a7de1551aa32192253dd0033a18c877e877c9960343a5b07c
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