Fix cmdline argument check

There was a segfault when sent was called without arguments. Now we use stdin
when there's no argument or - is used. Thanks to izabera for the report.

1 files changed, 5 insertions, 5 deletions

diff --git a/README.md b/README.md
index 70f60f9..3583d33 100644
--- a/README.md
+++ b/README.md
@@ -25,12 +25,12 @@ You can navigate with the arrow keys and quit with `q`.
 
 Usage
 
-	sent FILE
+	sent [FILE]
 
-If FILE equals `-`, stdin will be read. Produce image slides by prepending a
-`@` in front of the filename as a single paragraph. Lines starting with `#` will
-be ignored. A `\` at the beginning of the line escapes `@` and `#`. A
-presentation file could look like this:
+If FILE is omitted or equals `-`, stdin will be read. Produce image slides by
+prepending a `@` in front of the filename as a single paragraph. Lines starting
+with `#` will be ignored. A `\` at the beginning of the line escapes `@` and
+`#`. A presentation file could look like this:
 
 	sent