Add notes on CMake flags for fetched plugins

Author: bakpaul

10_Getting_Started/20_Build/50_Activate_Plugins.md

@@ -15,6 +15,17 @@ This page aims at summarizing those extensions in tables containing:
 
 The activation directives propose two ways of activating those plugins in the build tree:
 
-1. Through custom CMake flags that [can be added during the CMake call](https://cmake.org/cmake/help/latest/manual/cmake.1.html#cmdoption-cmake-D). All of those plugins can be activated with one or two CMake flags named using their type and name. For example, the plugin SofaPython3 can be activated by activating both flags `SOFA_FETCH_SOFAPYTHON3=ON` and `PLUGIN_SOFAPYTHON3=ON` (because its sources are in a separate repository).
+1. Through custom CMake flags that [can be added during the CMake call](https://cmake.org/cmake/help/latest/manual/cmake.1.html#cmdoption-cmake-D). All of those plugins can be activated with one or two CMake flags named using their type and name. For example, the plugin SofaPython3 can be activated by activating both flags `SOFA_FETCH_SOFAPYTHON3=ON` and `PLUGIN_SOFAPYTHON3=ON` (because its sources are in a separate repository, see bellow).
 2. Through preset that can be [specified during the CMake call](https://cmake.org/cmake/help/latest/manual/cmake-presets.7.html#introduction)
 
+### Note on fetched plugins
+
+As stated before, some plugins have their own repository and thus need to e fetched to be compiled in-tree. To do so, you'll need to activate the following flag `SOFA_FETCH_XXX=ON` while replacing `XXX` by the capitalized name of your plugin (note: all '.' in the name shuold be replaced by a '_', e.g. to fetch Sofa.Qt, one need to set the CMake flag `SOFA_FETCH_SOFA_QT=ON`).
+
+The repository from which to fetch and the tag to checkout can be changed by using the two flags `XXX_GIT_REPOSITORY` and `XXX_GIT_TAG`. The tag can be either a tag, a branch name or a commit hash. 
+
+It has to be noted that you can use a local clone of the repository instead of rely on the fetching mechanism. This can be usefull when you have a version of the plugin on your disk on which you are currently working. To do this, you'll need to make sure `SOFA_FETCH_XXX` is `OFF` and then you can specify the following flag `XXX_LOCAL_DIRECTORY` with an abolute path to the local clone of the plugin. 
+
+No matter what mechanism you use (fetch or local clone) you'll still need to activate it by activating the following CMake flag `{TYPE}_XXX` with type taken into the list `{"APPLICATION", "PLUGIN", "DIRECTORY"}`.
+
+