Add meson build guide

contrib/meson/README

 This Meson project is provided with no guarantee and maintained by Dima Krasner <dima@dimakrasner.com>.
 
 It outputs one libzstd, either shared or static, depending on default_library.
+
+How to build
+============
+
+`cd` to this meson directory (`zstd/contrib/meson`) and type:
+
+    $ meson --buildtype=release --strip --prefix=/usr builddir
+    $ ninja             # to build
+    $ ninja install     # to install