[boringssl] Updating rolling instructions

Update the instructions on the bindgen dependency as people
frequently trip over it.

Change-Id: I49bbef2f95884a15034cfc4bae9b1838cf6997df
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/boringssl/+/615240
Fuchsia-Auto-Submit: Ali Zhang <alizhang@google.com>
Reviewed-by: Allison Pearce <ampearce@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
diff --git a/README.fuchsia b/README.fuchsia
index f5bb3d1..9ac75fa 100644
--- a/README.fuchsia
+++ b/README.fuchsia
@@ -14,7 +14,10 @@
 
 To update:
 1. `rustup update`
-1. `cargo install bindgen`
+1. `cargo install bindgen` and update BINDGEN_EXPECTED_VERSION in
+   ./rust/boringssl-sys/bindgen.sh to `bindgen --version`. (If things regress,
+   revert back to pinned version with `cargo install bindgen --version
+   ${BINDGEN_EXPECTED_VERSION}` and investigate).
 1. `go run roll_boringssl.go`
 1. Test according to instructions given by the previous step.
 1. Commit, review, and submit the changes to this repository (you may need to