add direct cargo build instruction

README.md

@@ -6,6 +6,12 @@ A file and text uploading service with configurable time limit
 
 ## compiling
 
+```sh
+cargo build --release
+```
+
+or
+
 ```sh
 docker build -t datatrash .
 docker cp datatrash:/home/rust/.cargo/bin/datatrash datatrash