Improve readme with usage

README.md

@@ -1,6 +1,20 @@
 # Primes counter
-````shell
+```shell
 go build
-./go-primes-counter -h
 ./go-primes-counter -start 10 -max 1000 -print-count -1 -step-size 100
-````
+```
+
+## Usage
+```text
+$ ./go-primes-counter -h
+Usage of ./go-primes-counter:
+  -max int
+    	The end value (default 10000)
+  -print-count int
+    	Number of primes to print. -1 to print all.
+    	Primes are not guaranteed to be sorted. (default 20)
+  -start int
+    	The start number
+  -step-size int
+    	The job size (default 1000)
+```