Add warning to those using master branch (#810)

Author: sbc100

emsdk.py

@@ -2842,6 +2842,18 @@ def expand_sdk_name(name, activating):
 
 
 def main(args):
+
+  if is_emsdk_sourced_from_github():
+    # This code only exists on the master branch
+    errlog('****')
+    errlog('Warning: You appear to be using the `master` branch of emsdk.')
+    errlog('We recently made the switch to using `main`')
+    errlog('In order to continue to receive updates you will need to make the switch locally too.')
+    errlog('For normal clones without any local branches simply running the following command should be enough:')
+    errlog('  `git checkout main`')
+    errlog('For more information see https://github.com/emscripten-core/emsdk/issues/805')
+    errlog('****')
+
   if not args:
     errlog("Missing command; Type 'emsdk help' to get a list of commands.")
     return 1