doc: add lockfile-path unstable doc section

Author: ifropc

src/doc/src/reference/unstable.md

@@ -112,6 +112,8 @@ Each new feature described below should explain how to use it.
 * Registries
     * [publish-timeout](#publish-timeout) --- Controls the timeout between uploading the crate and being available in the index
     * [asymmetric-token](#asymmetric-token) --- Adds support for authentication tokens using asymmetric cryptography (`cargo:paseto` provider).
+* Mafinest and lockfile 
+    * [lockfile-path](#lockfile-path) --- Allows to specify a path to lockfile other than the default path `<workspace_root>/Cargo.lock`.   
 * Other
     * [gitoxide](#gitoxide) --- Use `gitoxide` instead of `git2` for a set of operations.
     * [script](#script) --- Enable support for single-file `.rs` packages.
@@ -1625,6 +1627,23 @@ will prefer the value in the configuration. The allows Cargo to add new built-in
 path bases without compatibility issues (as existing uses will shadow the
 built-in name).
 
+## lockfile-path
+* Original Issue: [#5707](https://github.com/rust-lang/cargo/issues/5707)
+* Tracking Issue: [#14421](https://github.com/rust-lang/cargo/issues/14421)
+
+This feature allows you to specify the path of lockfile Cargo.lock. 
+By default, lockfile is written into `<workspace_root>/Cargo.lock`. 
+However, when sources are stored in read-only directory, most of the cargo commands 
+would fail, trying to write a lockfile. The `--lockfile-path`
+flag makes it easier to work with readonly sources. 
+Note, that currently path must end with `Cargo.lock`. Meaning, if you want to use 
+this feature in multiple projects, lockfiles should be stored in different directories.
+Example:
+
+```sh
+cargo +nightly metadata --lockfile-path=$LOCKFILES_ROOT/my-project/Cargo.lock -Z unstable-options
+```
+
 # Stabilized and removed features
 
 ## Compile progress