Add block storage service documentation

Adds a service description for the block storage service.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I3300511f0bdd68b32cb8546332177ae6959c7b29
diff --git a/docs/developer/service-descriptions/index.rst b/docs/developer/service-descriptions/index.rst
index 6574a3e..9814958 100644
--- a/docs/developer/service-descriptions/index.rst
+++ b/docs/developer/service-descriptions/index.rst
@@ -8,6 +8,7 @@
     attest-service-description
     crypto-service-description
     secure-storage-service-description
+    block-storage-service-description
     uefi-smm-services
 
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