Document MIRI_AUTO_OPS

Author: oli-obk

README.md

@@ -393,6 +393,9 @@ Some native rustc `-Z` flags are also very relevant for Miri:
 
 Moreover, Miri recognizes some environment variables:
 
+* `MIRI_AUTO_OPS` indicates whether the automatic execution of rustfmt, clippy and rustup-toolchain
+  should be skipped. If it is set to any value, they are skipped. This is used for avoiding
+  infinite recursion in `./miri` and to allow automated IDE actions to avoid the auto ops.
 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
   Miri executions, also [see "Testing the Miri driver" in `CONTRIBUTING.md`][testing-miri].
 * `MIRIFLAGS` (recognized by `cargo miri` and the test suite) defines extra