Add feature documentation to manifest

In order to assist users of the library understanding which features
should be enabled for what functionality add documentation to the
optional dependencies.

Author: tcharding

Cargo.toml

@@ -25,8 +25,11 @@ serde-std = ["serde/std"]
 unstable = []  # for benchmarking
 
 [dependencies]
+# Only enable this if you explicitly do not want to use "std", otherwise enable "serde-std".
 serde = { version = "1.0", default-features = false, optional = true }
+# Can only be used with "std" enabled.
 schemars = { version = "<=0.8.3", optional = true }
+# Only enable this if you explicitly do not want to use an allocator, otherwise enable "alloc".
 core2 = { version = "0.3.0", optional = true, default_features = false }
 
 [dev-dependencies]