Migrate instructions need to be clearer - inform users to overwrite old `margo.go` file.

[zvodd]

I recently migrated to the dev branch from the package control package. After following the instructions at https://margo.sh/b/migrate/ margo.go builds kept failing with : src\margo\margo.go:86:21: undefined: mg.Reduce and margo.sublime not found in %PATH% . I finally found a solution in DisposaBoy's comment on issue #858 I am idiot - this took FAR TO LONG!

Turns out I had the margo.go source file from the old branch. I really think the install / migrate instructions should clearly warn migrating users to overwrite this file. E.g. big red text: "User migrating to dev branch from Package Control NEED to replace $packages\User\GoSublime\src\margo\margo.go with https://github.com/KurokuLabs/margo/blob/master/extension-example/extension-example.go before editing."

[DisposaBoy]

The margo.go file should only be present if you previously manually switched to the new margo implementation, the development branch just happens to have some API breakages.

And in that case, I mention it after the Restart Sublime Text. step:

If you've already switched to the new margo implementation, it might help to check the extension example for changes as there are some API breakages.