docs: add description & README.md to mithril-build-script crate

Author: Damien LACHAUME / PALO-IT

mithril-build-script/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 name = "mithril-build-script"
 version = "0.1.0"
+description = "A toolbox for Mithril crates build scripts"
 authors = { workspace = true }
 edition = { workspace = true }
 homepage = { workspace = true }

mithril-build-script/README.md

@@ -0,0 +1,5 @@
+# Mithril-build-script
+
+A toolbox for Mithril crates that need a [build scripts phase](https://doc.rust-lang.org/cargo/reference/build-scripts.html).
+
+Moreover, this crate allows to test the build scripts, which is not supported directly in `build.rs` (see: [rust-lang/cargo#1581](https://github.com/rust-lang/cargo/issues/1581)).