Mention spurious CI failures during publish

Cargo.toml

@@ -46,4 +46,10 @@ travis-ci = { repository = "alexcrichton/proc-macro2" }
 # section. You need to delete this section and then publish with --allow-dirty.
 # A PR https://github.com/rust-lang/cargo/pull/6535 proposes to make it no
 # longer an error to publish with a patch section.
+#
+# Travis builds that are in progress at the time that you publish may spuriously
+# fail. This is because they'll be building a local proc-macro2 which carries
+# the second-most-recent version number, pulling in quote which resolves to a
+# dependency on the just-published most recent version number. Thus the patch
+# will fail to apply because the version numbers are different.
 proc-macro2 = { path = "." }