minor: suggest workarounds for rustc 1.46+ regression (mongodb#270)

Author: patrickfreed

Diff for: README.md

@@ -36,6 +36,11 @@ This repository contains the officially supported MongoDB Rust driver, a client
 | 0.9.x          | 1.39+                 |
 - MongoDB 3.6+
 
+**Note**: A regression introduced in Rust 1.46 may cause out-of-memory errors when compiling an application that uses
+the driver with a framework like actix-web. Rust 1.45 or the latest nightly version can be used to work around this
+problem temporarily. For more information or to track progress on a fix, see
+https://github.com/rust-lang/rust/issues/75992.
+
 ### Importing
 The driver is available on [crates.io](https://crates.io/crates/mongodb). To use the driver in your application, simply add it to your project's `Cargo.toml`.
 ```toml