Add env var to docs and update Readme

Author: jishnub

README.md

@@ -37,10 +37,24 @@ This package performs some type piracy and is also included in the sysimage, whi
 To use a development version of this package, you can choose one of the following methods:
 
 1. **Change the UUID in the project file and load the package:**
-   This approach will produce warnings and may lead to method ambiguities between the development version and the one in the sysimage, but it can be used for basic experimentation.
+
+   Start `julia` as
+   ```julia
+   JULIA_PRUNE_OLD_LA=true julia +nightly --compiled-modules=existing --project
+   ```
+   where it is assumed that one is already within the `LinearAlgebra` directory. Otherwise, adjust
+   the project path accordingly. The `julia +nightly` command above assumes that `juliaup` is being used
+   to launch `julia`, but one may substitute this with the path to the julia executable.
+
+   Within the `julia` session, load the `LinearAlgebra` module after pruning the one in the sysimage. This may be done as
+   ```julia
+   include("test/prune_old_LA.jl") && using LinearAlgebra
+   ```
 
 2. **Build Julia with the custom `LinearAlgebra` commit:**
-   Modify the commit in `stdlib/LinearAlgebra.version` and build Julia.
+
+   Modify the commit in [`stdlib/LinearAlgebra.version`](https://github.com/JuliaLang/julia/blob/master/stdlib/LinearAlgebra.version) and build Julia. This requires one to push the development branch
+   to `GitHub` or an equivalent platform.
 
 3. **Build a custom sysimage with the new `LinearAlgebra`:**
    - Install `PackageCompiler`.

docs/make.jl

@@ -1,3 +1,4 @@
+ENV["JULIA_PRUNE_OLD_LA"] = "true"
 include("../test/prune_old_LA.jl")
 
 using LinearAlgebra