docs: add alias to plugin add/install

Author: KristianDD

docs/man_pages/lib-management/plugin-add.md

@@ -8,6 +8,7 @@ position: 1
 Usage | Synopsis
 ------|-------
 General | `$ tns plugin add <Plugin>`
+Alias | `$ tns plugin install <Plugin>`
 
 <% if(isConsole) { %>Installs the specified plugin and any packages that it depends on.<% } %>
 <% if(isHtml) { %>Installs the specified plugin and its dependencies in the local `node_modules` folder, adds it to the `dependencies` section in `package.json`, and prepares the plugin for all installed platforms. If you have not configured any platforms for the project, the NativeScript CLI will prepare the plugin when you add a platform. For more information about working with plugins, see [NativeScript Plugins](https://github.com/NativeScript/nativescript-cli/blob/master/PLUGINS.md).<% } %>

docs/man_pages/lib-management/plugin-install.md

@@ -8,6 +8,7 @@ position: 3
 Usage | Synopsis
 ------|-------
 General | `$ tns plugin install <Plugin>`
+Alias | `$ tns plugin add <Plugin>`
 
 <% if(isConsole) { %>Installs the specified plugin and any packages that it depends on.<% } %>
 <% if(isHtml) { %>Installs the specified plugin and its dependencies in the local `node_modules` folder, adds it to the `dependencies` section in `package.json`, and prepares the plugin for all installed platforms. If you have not configured any platforms for the project, the NativeScript CLI will prepare the plugin when you add a platform. For more information about working with plugins, see [NativeScript Plugins](https://github.com/NativeScript/nativescript-cli/blob/master/PLUGINS.md).<% } %>