Docs: Add key features for nim

Closes exercism#270

config.json

@@ -931,7 +931,38 @@
     ]
   },
   "concepts": [],
-  "key_features": [],
+  "key_features": [
+    {
+      "title": "Strong Static Typing",
+      "content": "Modern statically typed system with type inference, tuples, generics and sum types.",
+      "icon": "statically-typed"
+    },
+    {
+      "title": "Fast",
+      "content": "Compiles to C/C++/ObjC with no run time or virtual machine for blazing fast results.",
+      "icon": "fast"
+    },
+    {
+      "title": "Expressive",
+      "content": "A python-like friendly sync and  macro system and generics to allow for easy DSL creation and easy code reusability.",
+      "icon": "expressive"
+    },
+    {
+      "title": "Extensible",
+      "content": "Easy FFI with the target backend language (C/C++/ObjC/Javascript) to allow for execellent and easy usage of preexisting libraries.",
+      "icon": "extensible"
+    },
+    {
+      "title": "Cross-platform",
+      "content": "Compiles anywhere a C compiler compiles to, allowing for usage on Windows/Linux/macOS/Android/iOS as well as embedded devices. Cross compilation to any other target platform",
+      "icon": "cross-platform"
+    },
+    {
+      "title": "Customizable Memory Management",
+      "content": "Automatic overloadable reference counting and a cycle detector to allow for no garbage collection at all.",
+      "icon": "garbage-collected"
+    }
+  ],
   "tags": [
     "paradigm/imperative",
     "paradigm/procedural",