add a README

(cherry picked from commit b7f82ec)

Author: KristofferC

README.md

@@ -0,0 +1,47 @@
+# LinearAlgebra
+
+This package is part of the Julia standard library (stdlib).
+
+`LinearAlgebra.jl` provides functionality for performing linear algebra operations in Julia.
+
+## Migrating a Pull Request (PR) from the Julia repository to this repository
+
+Since this package was split out from the main Julia repository, you might have previously made a pull request (PR) to the Julia repo. You can easily migrate such PRs to this repository using the following steps:
+
+1. Add the Julia repository (or your fork) as a new remote repository:
+   ```bash
+   git remote add juliarepo https://github.com/JuliaLang/julia
+   ```
+
+2. Fetch the commits from the Julia repository:
+   ```bash
+   git fetch juliarepo
+   ```
+
+3. Cherry-pick the relevant commits made in the Julia repository to this repository:
+   ```bash
+   git cherry-pick $JULIA_COMMIT
+   ```
+
+## Using development versions of this package
+
+This package performs some type piracy and is also included in the sysimage, which makes using a development version slightly more complex than usual.
+
+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.
+
+2. **Build Julia with the custom `LinearAlgebra` commit:**
+   Modify the commit in `stdlib/LinearAlgebra.version` and build Julia.
+
+3. **Build a custom sysimage with the new `LinearAlgebra`:**
+   - Install `PackageCompiler`.
+   - Load it and, with this project active, run:
+     ```julia
+     create_sysimage(["LinearAlgebra"]; sysimage_path="new_sysimage.so", incremental=false, filter_stdlibs=true)
+     ```
+   - Start Julia with the custom sysimage:
+     ```bash
+     julia -Jnew_sysimage.so
+     ```