Update readme with debug/int64 instructions

Author: tdrwenski

README.md

@@ -55,7 +55,8 @@ pip install -e .
 #### Optional Spack environment variations
 The Spack environment used to build Quandary is defined in `.spack_env/spack.yaml`.
 You can add or remove packages from the `specs` list as needed or use different variants of these. 
-For instance, if you want to use the debug variant (which builds Petsc in debug mode) you can use `quandary@develop+test+debug`.
+For instance, if you want to use the debug variant (which builds Quandary and Petsc in debug mode) you can use `quandary@develop+test build_type=Debug`.
+To build with `PetscInt`s set to 64-bit integers, use `quandary+int64`.
 To use a specific version of Petsc instead of the latest release, you can do e.g. `quandary@develop^petsc@3.22.1`.
 The `+test` variant (by default on in `.spack_env/spack.yaml`) adds python and pip to the Spack environment.
 This allows `pip install` to install python packages to your Spack virtual environment rather than a global one.
@@ -89,6 +90,7 @@ cmake ..
 make
 ```
 
+To build in debug mode use `cmake -DCMAKE_BUILD_TYPE=Debug ..`.
 Add the path to Quandary to your `PATH` variable with `export PATH=/path/to/quandary/:$PATH`, so your binary can be found.
 Alternatively, you can install the Quandary executable in a specific path (such as the default `/usr/local/bin` to have it in your `PATH` automatically):
 ```