doc: add info about options to the readme

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "vnetod"
 description = "Dotenv state switcher"
-version = "0.2.0"
+version = "0.2.1"
 edition = "2021"
 license = "GPL-3.0+"
 repository = "https://git.pleshevski.ru/pleshevskiy/vnetod.git"

README.md

@@ -25,6 +25,21 @@ vnetod local debug  # enable local and debug sections
 vnetod              # disable all sections
 ```
 
+This tool uses `.env` from your current location, but you can change this
+behavior with the `-f` (`--file`) flag.
+
+```sh
+cp .env.example .env.properties
+vnetod .env.properties local
+```
+
+And you can also change the output file with the `-o` (`--output`) flag, if you
+don't want to overwrite the input file.
+
+```sh
+vnetod -f .env.example -o .env local
+```
+
 You can also use variables from namespaces
 
 ```sh
@@ -37,6 +52,12 @@ You can switch between states and overwrite from namespaces at the same time.
 vnetod local db:staging debug:off
 ```
 
+For more information, see the help.
+
+```sh
+vnetod --help
+```
+
 # License
 
 GNU General Public License v3.0 or later