Add metadata file.

meta/libraries.json

@@ -0,0 +1,14 @@
+{
+    "key": "xpressive",
+    "name": "Xpressive",
+    "authors": [
+        "Eric Niebler"
+    ],
+    "description": "Regular expressions that can be written as strings or as expression templates, and which can refer to each other and themselves recursively with the power of context-free grammars.",
+    "category": [
+        "String"
+    ],
+    "maintainers": [
+        "Eric Niebler <eric -at- boostpro.com>"
+    ]
+}