Add stdin documentation.

2 files changed, 7 insertions, 0 deletions

diff --git a/README.md b/README.md
index 9d4ccf0..a53fe44 100644
--- a/README.md
+++ b/README.md
@@ -15,6 +15,11 @@ scripts that can output in that format.
     $ spark 0 30 55 80 33 150
     ▁▂▄▅▃█
 
+If no arguments are passed, it will read from stdin a set of numbers delimited by any character except 0 through 9, - (dash), . (period), e, and E.
+
+    $ echo "12,1 6 8*3" | spark
+    █▂▅▆▃
+
 Invoke help with `spark -h`.
 
 ## Example
diff --git a/spark.c b/spark.c
index 853a637..789b813 100644
--- a/spark.c
+++ b/spark.c
@@ -11,6 +11,8 @@ int main(int argc, char *argv[])
 	if (isatty(fileno(stdin)) && (argc < 2 || !strcmp(argv[1], "-h") || !strcmp(argv[1], "--help"))) {
 		fprintf(stderr, "Spark\n  by Jason A. Donenfeld <Jason@zx2c4.com>\n\n");
 		fprintf(stderr, "Usage: %s [number] [number] [number] ...\n", argv[0]);
+		fprintf(stderr, "If no arguments are given, read from stdin numbers delimited by\n");
+		fprintf(stderr, "any characters except 0 1 2 3 4 5 6 7 8 9 . - e E.\n");
 		return 1;
 	}
 	double *values;