Improve plugin compilation and use

tools/plugin-template/README.md-template

@@ -11,9 +11,9 @@ Building this plugin requires an **installed** version of `pugs`.
 Before building `pugs` one should define its installation directory.
 In the `pugs` compilation directory one should execute
 ```shell
-cmake -DCMAKE_INSTALL_PREFIX=pugs_install_dir ...
+cmake -DCMAKE_INSTALL_PREFIX=[pugs_install_dir] [pugs_src_path] [...]
 ```
-where `pugs_install_dir` is the chosen installation directory.
+where `[pugs_install_dir]` is the chosen installation directory.
 
 Then one simply runs
 ```shell
@@ -30,12 +30,12 @@ make install
 > is not able to remove them, to avoid the risk of compilation issues,
 > one has to dot it manually...
 
-In the build directory one runs
+In the plugin build directory one runs
 ```shell
-cmake -DPUGS_PREFIX_PATH=pugs_install_dir _PLUGIN_LOW__dir
+cmake -DPUGS_PREFIX_PATH=[pugs_install_dir] [_PLUGIN_LOW__src_dir]
 ```
-where `pugs_install_dir` has the same value as above and `_PLUGIN_LOW__dir`
-is the directory that contains this `README.md` file.
+where `[pugs_install_dir]` has the same value as above and `[_PLUGIN_LOW__src_dir]`
+is the plugin source directory that contains this `README.md` file.
 
 Then to build the plugin, one runs
 ```shell
@@ -65,3 +65,6 @@ export PUGS_PLUGIN_DIR="/pathtoplugin1;/pathtoplugin2"
 ```
 
 Then one launches `pugs` classically.
+```shell
+[pugs_install_dir]/bin/pugs [script.pgs]
+```