add a note about getopts constraints

KodrAus · web-flow · commit 70e114c38ce6 · 2020-07-19T11:10:00.000+10:00
diff --git a/README.md b/README.md
@@ -13,3 +13,7 @@ Add this to your `Cargo.toml`:
 [dependencies]
 getopts = "0.2"
 ```
+
+## Contributing
+
+The `getopts` library is used by `rustc`, so we have to be careful about not changing its behavior.
