Add contributing manual

CONTRIBUTING.md

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+  - This repository follows the [SciMLStyle](https://github.com/SciML/SciMLStyle) and the SciML [ColPrac](https://github.com/SciML/ColPrac).
+  - Please run `using JuliaFormatter, NonlinearSolve; format(joinpath(dirname(pathof(NonlinearSolve)), ".."))` before committing.
+  - Add tests for any new features.
+  - Additional help on contributing to the numerical solvers can be found at https://docs.sciml.ai/NonlinearSolve/stable/
+
+## Developing Locally
+
+NonlinearSolve is a large package and thus it uses a sublibrary approach to keep down
+the total number of dependencies per solver. As a consequence, it requires a bit of special
+handling compared to some other Julia packages.
+
+When running the subpackage test suite, it's recommended that one has dev'd its relevant packages in NonlinearSolve.jl. This can be done via(use `NonlinearSolveFirstOrder` as example):
+
+```julia
+using Pkg
+dev_pks = Pkg.PackageSpec[]
+for path in ("lib/SciMLJacobianOperators", "lib/NonlinearSolveBase")
+    push!(dev_pks, Pkg.PackageSpec(; path))
+end
+Pkg.develop(dev_pks)
+```
+
+When running the full test suite, it's
+recommended that one has dev'd all of the relevant packages. This can be done via:
+
+```julia
+using Pkg
+Pkg.develop(map(
+    path -> Pkg.PackageSpec.(; path = "$(@__DIR__)/lib/$(path)"), readdir("./lib")));
+```
+
+and then running tests accordingly.
+
+## Dependency Structure
+
+There is a tree dependency structure to the sublibraries of NonlinearSolve.jl. The core subpackage NonlinearSolveBase.jl is the hard dependency of all the other subpackages. SimpleNonlinearSolve.jl is a special one with BracketingNonlinearSolve.jl as dependency. NonlinearSolve.jl as the parent package contains all of the subpackages.