Add an example of PATH customisation on Windows

On Windows you often find directories containing spaces. The relation
between quotes and escaping is not intuitive, see #765.

docs/options.md

@@ -499,6 +499,10 @@ Platform-specific environment variables are also available:<br/>
     # Append a directory to the PATH variable (this is expanded in the build environment)
     CIBW_ENVIRONMENT: "PATH=$PATH:/usr/local/bin"
 
+    # Prepend a directory containing spaces on Windows.
+    CIBW_ENVIRONMENT_WINDOWS: >-
+      PATH="C:\\Program Files\\PostgreSQL\\13\\bin;$PATH"
+
     # Set BUILD_TIME to the output of the `date` command
     CIBW_ENVIRONMENT: "BUILD_TIME=$(date)"