docs: add features.md for feature explanation

nixd/docs/features.md

@@ -0,0 +1,57 @@
+## Features in nixd
+
+
+### Semantic Tokens
+
+[nixd language server](https://github.com/nix-community/nixd) tries to make some tokens looks different by sending your editor some integers.
+However, types in nix language is pretty different from standard LSP types.
+So, as a result, attrnames, selection, variables are colored as different integers,
+but the colors may, or may not rendered properly in your editor.
+
+#### Attribute name coloring
+
+Colors may be different for attribute path creating nested attribute set between the path just "select"s into it.
+
+```nix
+{
+    foo.bar = 1;
+#   ~~~ ~~~ <----- these two tokens should be colored as same
+    foo.x = 2;
+#   ~~~     <----- this token, however will not be colored as same as "foo.bar"
+}
+```
+
+For the attribute path `foo.bar`, it creates "nested" attribute set, under "foo".
+
+```nix
+{
+    foo = {
+#   ^~~  <-------- new !
+        bar = 1;
+        #  <------- new !
+    };
+}
+```
+
+And the second, `foo.x`, just make a new "entry" in "foo"
+
+```nix
+{
+    foo = {
+        bar = 1;
+        x = 2;
+        # <----  new !
+    };
+}
+```
+
+For example in my editor setup they looks like:
+
+<img width="566" alt="semantic tokens, bootspec" src="https://github.com/nix-community/nixd/assets/36667224/18072790-6ded-4ab9-aaa9-022a428e5da7">
+
+
+Thus you can only look at "darker" attrset, which really creates something different.
+
+For example in this picture you can only focus `.availableKernelModules`, in `boot.initrd.availableKernelModules`.
+
+