add description about editing poac.toml directly to add dependency

docs/usage.md

@@ -50,7 +50,9 @@ Poac uses a cache since we executed the command with no changes.
 
 ## Install dependencies
 
-Like Cargo does, Poac installs dependencies at build time.  Poac currently supports Git and system dependencies.  You can use the `poac add` command to add dependencies to your project.
+Like Cargo does, Poac installs dependencies at build time.  Poac currently supports Git, path, and system dependencies.  You can use two methods to add dependencies to your project: the `poac add` command, and editing `poac.toml` directly.
+
+### `poac add`
 
 The `poac add` command accepts the following arguments:
 
@@ -71,6 +73,26 @@ poac add "ToruNiina/toml11" --rev "846abd9a49082fe51440aa07005c360f13a67bbf"
 
 If `tag`, `branch`, or `rev` is unspecified for git dependencies, Poac will use the latest revision of the default branch. System dependency names must be acceptable by `pkg-config`. The version requirement syntax is specified in [src/VersionReq.hpp](https://github.com/poac-dev/poac/blob/main/src/VersionReq.hpp).
 
+### editing `poac.toml` directly
+
+The syntax for `poac.toml` like below:
+
+```toml
+[dependencies]
+
+# git dependency
+"ToruNiina/toml11" = { git = "https://github.com/ToruNiina/toml11.git", rev = "846abd9a49082fe51440aa07005c360f13a67bbf" }
+
+# path dependency
+local_lib = { path = "../local_lib" }
+
+# system dependency
+fmt = { version = ">= 9", system = true }
+```
+
+If `tag`, `branch`, or `rev` is unspecified for git dependencies, Poac will use the latest revision of the default branch. System dependency names must be acceptable by `pkg-config`. The version requirement syntax is specified in [src/VersionReq.hpp](https://github.com/poac-dev/poac/blob/main/src/VersionReq.hpp).
+
+
 After adding dependencies, executing the `build` command will install the package and its dependencies.
 
 ```console