docs: update main branch missing message

Author: arxanas

git-branchless-lib/src/core/repo_ext.rs

@@ -55,7 +55,9 @@ These branches exist: {:?}
 Either create it, or update the main branch setting by running:
 
     git branchless init --main-branch <branch>
-",
+
+Note that remote main branches are no longer supported as of v0.6.0. See
+https://github.com/arxanas/git-branchless/discussions/595 for more details.",
                     get_main_branch_name(self)?,
                     self.get_path(),
                     self.get_all_local_branches()?

git-branchless/tests/command/test_init.rs

@@ -338,6 +338,8 @@ fn test_main_branch_not_found_error_message() -> eyre::Result<()> {
 
         git branchless init --main-branch <branch>
 
+    Note that remote main branches are no longer supported as of v0.6.0. See
+    https://github.com/arxanas/git-branchless/discussions/595 for more details.
 
     Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
     Run with RUST_BACKTRACE=full to include source snippets.