Add upstream merge instruction

CONTRIBUTORS_GUIDE.md

@@ -57,4 +57,13 @@ Tests exist, but coverage is very limited. What's covered is a conjunction of
 (e.g. no automatic checks for renegotiations, parroting quality and such)
 
 plus we test some other minor things.
-Basically, current tests aim to provide a sanity check.
+Basically, current tests aim to provide a sanity check.
+
+# Merging upstream
+```Bash
+git remote add -f golang [email protected]:golang/go.git
+git checkout -b golang-upstream golang/master
+git subtree split -P src/crypto/tls/ -b golang-tls-upstream
+git checkout master
+git merge --no-commit golang-tls-upstream
+```