List OpenEXR 3.0.0 and higher as currently not supported in Unix install docs.

Author: c-lipka

unix/README.md

@@ -8,6 +8,9 @@ will need to run a pre-build step. This generates the configure script
 and does a few other things usually performed by our developers prior to
 releasing a source package.
 
+Should you run into problems, please consult `unix/install.txt` for more
+in-depth installation mojo.
+
 NB if you have improvements to the below instructions, please add a comment
 to issue #1, or (if you are working in a branch) send us a pull request.
 

unix/install.txt

@@ -141,6 +141,10 @@ recommanded to generate a fully-featured POV-Ray executable.
 
 - When using Boost 1.65 or above, the compiler must conform to C++11 or later.
 
+- OpenEXR version 3.0.0 or higher is currently not supported, nor even
+  properly tested for. We hope to change this in the future, but fixing it is
+  a bit more complicated.
+
 
 ====================================================
 3. Configuring, building, and (un)installing POV-Ray