Release 0.3.0

Bumps the version to 0.3.0 and keeps the roadmap notes out of the published
artifact. No source changes: this tags the BS3, Vern7, Rosenbrock23 and
PIDController work that has accumulated on main since 0.2.3.

Cargo.toml

@@ -1,12 +1,13 @@
 [package]
 name = "ordinary-diffeq"
-version = "0.2.3"
+version = "0.3.0"
 edition = "2021"
 authors = ["Connor Johnstone"]
 description = "A library for solving differential equations based on the DifferentialEquations.jl julia library."
 readme = "readme.md"
 repository = "https://gitlab.rcjohnstone.com/connor/differential-equations"
 license = "MIT"
+exclude = ["roadmap/"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html