Potential GDAL multi-version conflict clarification

[TMinRay]

While testing the GIS converter tool, I ran into an issue caused by having both:

• system GDAL (libgdal-dev, e.g., 3.8.x), and
• a source-built GDAL (e.g., 3.12.1 under /usr/local)

installed simultaneously.

When building the Python GDAL bindings, the compiler can mix headers from one installation with libraries from another, which leads to ABI mismatches (e.g., linking against libgdal.so.34 vs libgdal.so.38) and import failures such as _gdal_array errors.

It may be helpful to clarify in the README that only one GDAL development stack should be active during the Python binding build.

Happy to open a PR to update the documentation accordingly.