Update SGX test data readme

Change-Id: I4b5185850d75e5b20d4f0516b596e502f2cdb47b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388770Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814034}

content/test/data/sxg/README.md

@@ -13,4 +13,15 @@ run:
 go get -u github.com/WICG/webpackage/go/signedexchange/cmd/...
 ```
 
+If the script still can't find the command-line tools, you might need to add
+the go package directory to your path and try again:
+
+```
+# generate-test-sxgs.sh must be run inside //src/content/test/data/sxg
+#
+# See https://golang.org/cmd/go/#hdr-Compile_and_install_packages_and_dependencies
+# for the actual value of ~/go/bin.
+PATH=$PATH:~/go/bin/ ./generate-test-sxgs.sh
+```
+
 The revision of the tools used to generate the test files is `99155ec`.