Clarify README paragraph about features.

whitequark · whitequark · commit 081851d9555e · 2017-09-24T16:07:24.000Z
We can easily (although annoyingly) do IP fragmentation with
the assembler now, so I don't see *any* features that are apriori
impossible to implement anymore.
diff --git a/README.md b/README.md
@@ -12,9 +12,8 @@ and compiles on stable Rust 1.19 and later.
 
 ## Features
 
-_smoltcp_ is missing many widely deployed features, whether by design or simply because
-no one implemented them yet. To set expectations right, both implemented and omitted
-features are listed.
+_smoltcp_ is missing many widely deployed features, usually because no one implemented them yet.
+To set expectations right, both implemented and omitted features are listed.
 
 ### Media layer
 
