[nongnu] elpa/rust-mode c8ae5dedb3: mention optional features

[ELPA Syncer]

branch: elpa/rust-mode
commit c8ae5dedb3deddaf35f6345da894ae26d156bf27
Author: brotzeit <brotzeitmacher@gmail.com>
Commit: brotzeit <brotzeitmacher@gmail.com>

    mention optional features
---
 README.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/README.md b/README.md
index 40f7037404..85d3ae9f52 100644
--- a/README.md
+++ b/README.md
@@ -30,6 +30,7 @@
         - [cargo.el](#cargoel)
         - [cargo-mode](#cargo-mode)
         - [rustic](#rustic)
+    - [Optional features](#optional-features)
     - [For package maintainers](#for-package-maintainers)
         - [Tests](#tests)
     - [Contributing](#contributing)
@@ -233,6 +234,18 @@ 
post](https://www.badykov.com/emacs/2021/05/29/emacs-cargo-mode/).
 extending it with other features such as integration with LSP and with
 flycheck.
 
+## Optional features
+
+The features of the following files can be disabled with 
`rust-load-optional-libraries`.
+
+- rust-cargo.el
+- rust-compile.el
+- rust-playpen.el
+- rust-rustfmt.el
+
+They are disabled by default when you use rustic as it has its own 
implementations
+for those features.
+
 ## Customization
 
 `rust-cargo-default-arguments` set additional cargo args used for 
check,compile,run,test