Add D++ library.

vendor/DPP/docpages/install/install-xmake.md

@@ -0,0 +1,21 @@
+\page install-xmake Installing from xmake
+
+To install D++ on a project from XMake:
+
+- Ensure XMake [is correctly installed](https://xmake.io/#/guide/installation)
+- Create a new xmake project if you haven't already one, using `xmake init <project_name>`
+- Update the `xmake.lua` file by adding the `dpp` package, below the minimum configuration:
+
+~~~~~~~~~~~{.cmake}
+add_rules("mode.debug", "mode.release")
+
+add_requires("dpp")
+
+target("test-bot")
+    set_kind("binary")
+    add_files("src/*.cpp")
+
+    add_packages("dpp")
+~~~~~~~~~~~
+
+- Finally, run `xmake build` to download dependencies and build the project