Doc: add build related docs.

docs/source/development/build.md

+# CMake: building software
+There is a script `build.sh` to build the CEPCSW. Inside this script, it consists of three major steps:
+* configure: in this step, a *build directory* is created and the cmake reads `CMakeLists.txt` in *source code directory* and generates config files which will be used to build the project. 
+* build: in this step, the project is built according to the config files under *build directory*. 
+* install: in this step, the libraries, header files and python scripts are installed into *install directory*, which is `InstallArea`.
+
+The `cmake` command reads the `CMakeLists.txt` files recursively to produce the config files. As there are some common patterns to build the packages in the project, we use some macros from Gaudi framework.
+
+* `gaudi_add_library`: produce a shared library, which will be linked by other libraries.
+  * The source code files need to be added after `SOURCES` option.
+  * The link libraries need to be added after `LINK` option.
+* `gaudi_add_module`: produce a module library, which will be imported by Gaudi. Note, this library should not be linked by other libraries.
+  * Options `SOURCES` and `LINK` are same as `gaudi_add_library`.
+
+We also use some macros from CMake.
+* `target_include_directories`: specify the include directories when build a target. 
+* `install`: install the targets. 
+
+Examples could be found in CEPCSW. 
+
+For example, the `RecPFACyber` package uses all the macros:
+```cmake
+gaudi_add_library(CrystalCaloRecLib
+                  SOURCES src/Objects/CaloUnit.cc
+                          src/Objects/CaloHit.cc
+                          ...
+                  LINK    k4FWCore::k4FWCore
+                          GearSvc
+                          CrystalEcalSvcLib
+                          ...
+)
+
+gaudi_add_module(CrystalCaloRec
+                 SOURCES src/CyberPFAlg.cpp
+                         src/CyberDataCol.cpp
+                         ...
+                 LINK    CrystalCaloRecLib
+)
+
+target_include_directories(CrystalCaloRec PUBLIC
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>/include
+  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+
+install(TARGETS CrystalCaloRecLib CrystalCaloRec
+  EXPORT CEPCSWTargets
+  RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}" COMPONENT bin
+  LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}" COMPONENT shlib
+  COMPONENT dev)
+```
+

docs/source/index.rst

@@ -23,6 +23,7 @@ Contents
    :caption: Development Process
 
    development/overview.md
+   development/build.md
    development/doc.md
 
 .. toctree::