Add unstable features docs

Author: Sdoba16

doc/unstable-features.md

@@ -0,0 +1,17 @@
+# Unstable features
+
+Unstable features are experimental compiler capabilities. They may change
+or be removed before stabilization.
+
+## Viewing available unstable features
+
+Run `simc --help`; the features are listed under the `-Z` flag.
+
+## Enabling an unstable feature
+
+Pass `-Z <feature-name>` to `simc`.
+
+## Adding or stabilizing a feature
+
+See the rustdoc on `UnstableFeature` in `src/unstable.rs`; the procedures
+live next to the code they mutate, so they cannot drift.