Add rust-toolchain.toml file

This file tells cargo (and other tools, like vscode rust-analyzer
plugin) to use the nightly toolchain by default.

This is needed to correctly open the project in vscode with
rust-analyzer.

We should probably fix a nigthly version, but for now leave it as a
generic `nightly`.

Author: BigBigos

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "nightly"