docs: Add "unstable_features"

and remove the respective section from the README instead.

README.md

@@ -260,14 +260,6 @@ passthrough = [
 
 For more detailed documentation on which environment variables are automatically passed to the build environment, see [Environment Variable Passthrough](https://github.com/cross-rs/cross/wiki/Configuration#environment-variable-passthrough) on our wiki.
 
-### Unstable Features
-
-Certain unstable features can enable additional functionality useful to
-cross-compiling. Note that these are unstable, and may be removed at any
-time (particularly if the feature is stabilized or removed), and will
-only be used on a nightly channel.
-
-- `CROSS_UNSTABLE_ENABLE_DOCTESTS=true`: also run doctests.
 
 ### Mounting volumes into the build environment
 

docs/unstable_features.md

@@ -0,0 +1,9 @@
+Certain unstable features can enable additional functionality useful to
+cross-compiling. Note that these are unstable, and may be removed at any time
+(particularly if the feature is stabilized or removed), and will only be used
+on a nightly channel.
+
+Here is the list of currently available unstable features:
+
+- `CROSS_UNSTABLE_ENABLE_DOCTESTS`: enable or disable running doctests
+  (example: `true`)