Improve build instructions in README, test on Rust 1.36

.circleci/config.yml

@@ -57,7 +57,7 @@ jobs:
 
   test-stable:
     docker:
-      - image: circleci/rust:latest
+      - image: circleci/rust:1.36
     <<: *run_with_build_env_vars
     steps:
       - checkout

README.md

@@ -169,14 +169,19 @@ nginx and Lua do not work on Windows - you can track the progress on [this issue
 </details>
 
 ## Building
+[![Rustc Version 1.36+](https://img.shields.io/badge/rustc-1.36+-red.svg)](https://blog.rust-lang.org/2019/07/04/Rust-1.36.0.html)
 
 Wasmer is built with [Cargo](https://crates.io/), the Rust package manager.
 
+The Singlepass backend requires nightly, so if you want to use it,
+
 Set Rust Nightly:
 ```
 rustup default nightly
 ```
 
+Otherwise an up to date (see badge above) verison of stable Rust will work.
+
 And install Wasmer
 ```sh
 # checkout code