Document how to use a non-detected Kerberos lib

Author: simmel

README.txt

@@ -68,6 +68,16 @@ Easy Way
 
     $ pip install gssapi
 
+You can also use a specific Kerberos library by defining the `GSSAPI_*`
+environment variables:
+
+.. code-block:: bash
+
+    $ export GSSAPI_LINKER_ARGS="$(krb5-config --libs gssapi)"
+    $ export GSSAPI_COMPILER_ARGS="$(krb5-config --cflags gssapi)"
+    $ # Too many variants, search for GSSAPI_MAIN_LIB in setup.py
+    $ export GSSAPI_MAIN_LIB="$PATH_TO/lib/yourkerberoslib.ext"
+
 From the Git Repo
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