Add LICENSE and README.md. Set default concurrency to the number of

CPUs, and read stdin if no input files are specified.

README.md

@@ -4,6 +4,7 @@ A golang port of b3sum
 
 This is a simple `b3sum` command using `github.com/zeebo/blake3`.
 
+```
 Usage:
   b3sum [flags] [file]...
   -h	Prints help information
@@ -11,6 +12,7 @@ Usage:
     	The number of output bytes, prior to hex encoding (default 32)
   -p int
     	The maximum concurrency (defaults to the number of CPUs)
+```
 
 If no input files are specified, `b3sum` reads `stdin`.