Add performance note to README (Close #44)

README.md

@@ -15,6 +15,20 @@ assert(correct('GPL-3.0'), 'GPL-3.0-or-later')
 assert(correct('GPL-3.0', { upgrade: false }), 'GPL-3.0')
 ```
 
+## Performance Note
+
+This package load and processes the `spdx-license-ids`
+package, as well as a few other large arrays of strings,
+into global variables.  That can take a few milliseconds.
+
+If you'd prefer to postpone that processing until your
+program actually invokes the exported function, consider
+using `require()` or dynamic `import()` to load the package
+just before you invoke it.
+
+Special thanks to Vinicius Lourenço ([@H4ad](https://github.com/H4ad))
+for investigating load times.
+
 ## Contributors
 
 spdx-correct has benefited from the work of several contributors.