Add instructions of how to use pre-vendored 'rustc-src'

Author: seqre

src/bootstrap/bootstrap.py

@@ -1035,6 +1035,10 @@ def check_vendored_status(self):
                 eprint('       Run `cargo vendor {}` to initialize the '
                       'vendor directory.'.format(sync_dirs))
                 eprint('Alternatively, use the pre-vendored `rustc-src` dist component.')
+                eprint('You can download it from '
+                    'https://forge.rust-lang.org/infra/other-installation-methods.html#source-code')
+                eprint('and then place the vendor directory from the archive in the root')
+                eprint('of the rust project.')
                 raise Exception("{} not found".format(vendor_dir))
 
             if not os.path.exists(cargo_dir):