Update README.md (#30)

* Update README.md

Add info about flag and image etc

* Update README.md

README.md

@@ -20,7 +20,8 @@ You can get the binary from [releases](https://github.com/fortio/terminal/releas
 
 Or just run
 ```
-CGO_ENABLED=0 go install fortio.org/terminal/ansipixels/fps@latest
+CGO_ENABLED=0 go install fortio.org/terminal/ansipixels/fps@latest  # to install or just
+CGO_ENABLED=0 go run fortio.org/terminal/ansipixels/fps@latest  # to run without install
 ```
 
 or even
@@ -33,4 +34,13 @@ or
 brew install fortio/tap/fps
 ```
 
+Use the `-image` flag to pass a different image to load as background.
+
+Pass an optional `maxfps` as argument.
+
+E.g `fps -image my.jpg 60` will run at 60 fps with `my.jpg` as background.
+
+After hitting any key to start the measurement, you can also resize the window at any time and fps will render with the new size.
+Use `q` to stop.
+
 ![fps screenshot](fps_sshot.png)