Updated README include possible installation issue
@@ -28,6 +28,8 @@ In order to compile `CCL` you need a few libraries:
## Installing CLASS
CCL uses CLASS as one of the possible ways of computing the matter power spectrum. In order to communicate with CLASS, CCL must be linked to its library. Before installing CCL proper you must therefore install this library first. Since this process is not necessarily straightforward, we provide a python script `class_install.py` that automatically downloads and install the latest tagged stable version of CLASS. You should run this script (`python class_install.py`) before carrying out the next steps. By default, the script assumes that your main C compiler is `gcc`. If that's not the case, pass the name of your C compiler to the script via the command-line argument `--c_comp` (i.e. `python class_install.py --c_comp=[name of compiler]`). Type `python class_install.py -h` for further details.
+This procedure has one final caveat: if you already have a working installation of CCL, `class_install.py` may fail the first time you run it. This can be fixed by either simply running `class_install.py` a second time, or by starting from scratch (i.e. downloading or cloning CCL).
+
Note that, if you want to use your own version of CLASS, you should follow the steps described in the section "Compiling against an external version of CLASS" below.
## C-only installation
