doc: RFQ: husht command line and top-level behavior

README.md

@@ -23,3 +23,38 @@ into the Rust code.
 
 Documentation of specific syntax features of husht will be added as they are
 implemented.
+
+## Usage
+
+`husht [options] [sources]`
+
+Transforms some files from husht to Rust. The source specification `sources`
+is a list of files, directories or glob patterns, defaulting to `.`. A
+directory `dir` is interpreted the same way as the glob pattern `dir/**` --
+in other words, all files recursively within that directory. Glob patterns
+are matched and all matching files are added to the list of files to process.
+If there is a destination directory (see below), all files in the destination
+directory are ignored. Files that do not have extension `.toml` or `.hsh` are
+ignored. The former are copied unchanged; the latter are transformed per the
+husht language specification into Rust `.rs` files.
+
+Options:
+  - -h, --help -- Print a usage summary and exit
+  - -o, --destination [dir/file] -- Specifies the destination of the
+    transformed file(s). If the destination is a single file, the sources must
+    be as well, and the one file is transformed into the other. A destination
+    specification without an extension is assumed to be a directory. The
+    default destination is `rust`.
+
+    If the destination is a directory, then the transform of each source file
+    is written into that directory, preserving its relative path to its "root".
+    The root of a file added by virtue of a glob is the top level directory
+    of that glob pattern. The root of a file specified explicitly is the
+    current directory.
+
+For example, if the `src` directory contains `main.hsh`, `sub/crate.hsh`, and
+`Cargo.toml` (and there are no other .hsh or .toml files in the directory
+tree), then `husht src` would write `rust/main.rs`, `rust/sub/crate.rs`, and
+`rust/Cargo.toml`. On the other hand just `husht`, defaulting to `husht .`,
+would write `rust/src/main.rs`, `rust/src/sub/crate.rs`, and
+`rust/src/Cargo.toml`.