Add "see more" link to GHE-not-supported warning

I lived for several months thinking that support for caching action on GHE is just a matter of time, because it's such an important thing to have. Only today, I discovered that originally it was not planned at all. And that people already created some workarounds. So I hope that linking the issue from the warning message will save other people from what happened to me :-)

src/restore.ts

@@ -7,7 +7,9 @@ import * as utils from "./utils/actionUtils";
 async function run(): Promise<void> {
     try {
         if (utils.isGhes()) {
-            utils.logWarning("Cache action is not supported on GHES");
+            utils.logWarning(
+                "Cache action is not supported on GHES. See https://github.com/actions/cache/issues/505 for more details"
+            );
             utils.setCacheHitOutput(false);
             return;
         }

src/save.ts

@@ -12,7 +12,9 @@ process.on("uncaughtException", e => utils.logWarning(e.message));
 async function run(): Promise<void> {
     try {
         if (utils.isGhes()) {
-            utils.logWarning("Cache action is not supported on GHES");
+            utils.logWarning(
+                "Cache action is not supported on GHES. See https://github.com/actions/cache/issues/505 for more details"
+            );
             return;
         }