update docs

README.md

@@ -119,7 +119,8 @@ Run `xh help` for more detailed information.
 - `@` for including files in multipart requests e.g `[email protected]` or `[email protected];type=image/jpeg;filename=goodbye.jpg`.
 - `:` for adding or removing headers e.g `connection:keep-alive` or `connection:`.
 - `;` for including headers with empty values e.g `header-without-value;`.
-- `=@`/`:=@` for setting the request body's JSON or form fields from a file (`=@` for strings and `:=@` for other JSON types).
+
+`@` postfix can be added to `=`,`:=`,`==` and `:` to read the value from a file e.g `x-api-key:@api-key.txt`.
 
 The request body can also be read from standard input, or from a file using `@filename`.
 

src/cli.rs

@@ -310,6 +310,8 @@ pub struct Cli {
     ///   - header: to unset a header
     ///   - header; to add a header with an empty value
     ///
+    /// `@` postfix can be added to `=`,`:=`,`==` and `:` to read the value from a file e.g. `x-api-key:@api-key.txt`.
+    ///
     /// A backslash can be used to escape special characters (e.g. weird\:key=value).
     #[clap(value_name = "REQUEST_ITEM", verbatim_doc_comment)]
     raw_rest_args: Vec<String>,