Update README

README.md

@@ -1 +1,13 @@
-primes
+# Primes with rust
+
+A simple program that calculates prime numbers with multithreading. The resulting numbers are written to stdout and to the file primes.txt.
+
+## Usage
+
+```
+# compile
+cargo build
+
+# run
+./target/debug/primes [<start number>]
+```