docs: Document vendored-ssl

cargo-espflash/README.md

@@ -51,9 +51,15 @@ If you would like to flash from a Raspberry Pi using the built-in UART periphera
 cargo install cargo-espflash --features=raspberry
 ```
 
-[libuv]: (https://libuv.org/)
-[cargo-binstall]: (https://github.com/cargo-bins/cargo-binstall)
+By default, in Unix systems, we use the [`vendored-openssl` Cargo feature] which may require additional tools such as `perl` and `make`. To disable this feature, use:
+```
+OPENSSL_NO_VENDOR=1 cargo install cargo-espflash
+```
+
+[libuv]: https://libuv.org/
+[cargo-binstall]: https://github.com/cargo-bins/cargo-binstall
 [releases]: https://github.com/esp-rs/espflash/releases
+[`vendored-openssl` Cargo feature]: https://github.com/rust-lang/cargo#compiling-from-source
 
 ## Usage