docs: update README.md

Author: JyJyJcr

nginx-sys/README.md

@@ -14,8 +14,10 @@ nginx-sys = "0.5.0"
 
 ## Features
 
-- `vendored`: Enables the build scripts to download and build a copy of nginx
-  source and link against it.
+- `vendored`: Enables the build scripts to download and build a copy of nginx source and link against it.
+  This feature is enabled by default.
+- `alloc`: Enables the components using memory allocation. If no `std` flag, `alloc` crate is internally used instead. This flag is mainly for `no_std` build.
+- `std`: Enables the components using `std` crate. Currently the available components are same to when specifying `alloc` only.
   This feature is enabled by default.
 
 ## Output variables