[spp] [PATCH 3/5] docs: add intro in usecases

[Yasufumi Ogawa]

To explain overview of usecases, and why spp_primary is not described in
this chapter, add an introduction in the top of the chapter.

Signed-off-by: Yasufumi Ogawa <yasufum.o at gmail.com>
---
 docs/guides/api_ref/index.rst     |  2 ++
 docs/guides/commands/index.rst    |  2 ++
 docs/guides/design/impl/index.rst |  2 ++
 docs/guides/design/index.rst      |  2 ++
 docs/guides/usecases/index.rst    | 18 ++++++++++++++++++
 5 files changed, 26 insertions(+)

diff --git a/docs/guides/api_ref/index.rst b/docs/guides/api_ref/index.rst
index 490ec15..5619147 100644
--- a/docs/guides/api_ref/index.rst
+++ b/docs/guides/api_ref/index.rst
@@ -1,6 +1,8 @@
 ..  SPDX-License-Identifier: BSD-3-Clause
     Copyright(c) 2018-2019 Nippon Telegraph and Telephone Corporation
 
+.. _spp_api_ref_index:
+
 API Reference
 =============
 
diff --git a/docs/guides/commands/index.rst b/docs/guides/commands/index.rst
index ce67f4b..faaf50a 100644
--- a/docs/guides/commands/index.rst
+++ b/docs/guides/commands/index.rst
@@ -1,6 +1,8 @@
 ..  SPDX-License-Identifier: BSD-3-Clause
     Copyright(c) 2010-2014 Intel Corporation
 
+.. _spp_commands_index:
+
 SPP Commands
 ============
 
diff --git a/docs/guides/design/impl/index.rst b/docs/guides/design/impl/index.rst
index 53f1c6c..5e6aa1b 100644
--- a/docs/guides/design/impl/index.rst
+++ b/docs/guides/design/impl/index.rst
@@ -2,6 +2,8 @@
     Copyright(c) 2010-2014 Intel Corporation
 
 
+.. _spp_design_impl_index:
+
 Implementation
 ==============
 
diff --git a/docs/guides/design/index.rst b/docs/guides/design/index.rst
index f8d982d..3bf2c1d 100644
--- a/docs/guides/design/index.rst
+++ b/docs/guides/design/index.rst
@@ -2,6 +2,8 @@
     Copyright(c) 2010-2014 Intel Corporation
     Copyright(c) 2018-2019 Nippon Telegraph and Telephone Corporation
 
+.. _spp_design_index:
+
 Design
 ======
 
diff --git a/docs/guides/usecases/index.rst b/docs/guides/usecases/index.rst
index f67ea5d..538db87 100644
--- a/docs/guides/usecases/index.rst
+++ b/docs/guides/usecases/index.rst
@@ -1,9 +1,27 @@
 ..  SPDX-License-Identifier: BSD-3-Clause
     Copyright(c) 2019 Nippon Telegraph and Telephone Corporation
 
+.. _spp_usecases_index:
+
 Use Cases
 =========
 
+As described in :ref:`Design<spp_design_index>`,
+SPP has several kinds of secondary process for
+usecases such as simple forwarding to network entities, capturing or
+mirroring packets for monitoring, or connecting VMs or containers for
+Service Function Chaining in NFV.
+
+This chapter is focusing on explaining about each of secondary
+processes with simple usecases.
+Usecase of ``spp_primary`` is not covered here because it is almost
+similar to ``spp_nfv`` and no need to explain both.
+
+Details of usages of each process is not covered in this chapter.
+You can refer the details of SPP processes via CLI from
+:ref:`SPP Commands<spp_commands_index>`,
+or via REST API from :ref:`API Reference<spp_api_ref_index>`.
+
 .. toctree::
    :maxdepth: 1
    :numbered:
-- 
2.17.1