Docs: Emphasize Windows Path syntax for CMAKE

Use "/" instead of "\" when assigning CMAKE path variables.

Signed-off-by: Ken Liu <Ken.Liu@arm.com>
Change-Id: I2c186865c2404bcc45bd59ba90a21a6f993586fc
diff --git a/docs/getting_started/tfm_getting_started.rst b/docs/getting_started/tfm_getting_started.rst
index 35d4eda..c5f21fc 100644
--- a/docs/getting_started/tfm_getting_started.rst
+++ b/docs/getting_started/tfm_getting_started.rst
@@ -268,6 +268,10 @@
 
     .. group-tab:: Windows
 
+        .. important::
+            Use "/" instead of "\\" when assigning Windows paths to CMAKE
+            variables, for example, use "c:/build" instead of "c:\\\\build".
+
         .. code-block:: bash
 
             cd trusted-firmware-m