improved readme (#47)

Author: ascandone

README.md

@@ -16,3 +16,7 @@ The language server features include:
 - Hover on values
 - Detect document symbols
 - Go to definition
+
+### Build locally
+If you have the golang toolchain installed locally, you can run `go install ./internal/numscript/numscript.go` to build the cli from source, which you'll be able to run by executing the `numscript` command
+Otherwise, you can download it from this repo's [releases](https://github.com/formancehq/numscript/releases)