📝 docs(lib): add documentation for cull-gmail library

- create initial documentation file for cull-gmail library
- include installation instructions and usage steps

docs/lib.md

@@ -0,0 +1,23 @@
+## cull-gmail Library Documentation
+
+The `cull-gmail` library provides types to enable the culling of emails using the Gmail API including the following steps:
+- login to get authorization
+- backup the mailbox 
+- filtered lists of the contents
+- move email matching a filtered list to trash
+
+### Installation
+
+Add the library to your program's `Cargo.toml` using `cargo add`:
+
+```bash
+$ cargo add cull-gmail
+```
+
+Or by configuring the dependencies manually in `Cargo.toml`:
+
+```toml
+[dependencies]
+cull-gmail = "0.1.0"
+```
+