Created Notes About ccache (markdown)

Notes-About-ccache.md

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+# background
+ccache is a nice tool, but we certainly realized some issue of its behavior in this project, that could waste your time instead of save your time. So it's **default off** now, unless you manually enable it. 
+
+# issue that could appear if you enable it
+Just keep in mind that these issues could be made:
+- When you compiled with ccache enabled, and use git to merge/pull new changes, it can only detect a part of your change, and would compile fail. It cannot be fixed by `make clean`, you hvae to delete the build dir and start over.
+- Sometimes ccache cause linking stage took extremely long.
+- Unknown linking error until you `make clean` and start over again.
+
+# I want enable it anyway
+If you are aware of these issue and you are sure you are able to realized them, when they appear, follow this to enable ccache:
+
+## for make: 
+### turn on
+```
+(...other steps that in compile guide...)
+mkdir build
+cd build
+cmake -DUSE_CCACHE=ON ..   
+make -j
+(...other steps that in compile guide...)
+```
+### turn off
+```
+(...other steps that in compile guide...)
+mkdir build
+cd build
+cmake -DUSE_CCACHE=OFF ..   
+make -j
+(...other steps that in compile guide...)
+```
+### default behavior (off)
+```
+(...other steps that in compile guide...)
+mkdir build
+cd build
+cmake ..   
+make -j
+(...other steps that in compile guide...)
+```
+
+## for ninja:
+### turn on
+```
+(...other steps that in compile guide...)
+mkdir build
+cd build
+cmake -G Ninja -DUSE_CCACHE=ON .. 
+ninja
+(...other steps that in compile guide...)
+```
+### turn off
+```
+(...other steps that in compile guide...)
+mkdir build
+cd build
+cmake -G Ninja -DUSE_CCACHE=OFF .. 
+ninja
+(...other steps that in compile guide...)
+```
+### default behavior (off)
+```
+(...other steps that in compile guide...)
+mkdir build
+cd build
+cmake -G Ninja .. 
+ninja
+(...other steps that in compile guide...)
+```