Explain how to manually enable Serde support for num-rational and num-bigint

[adamreichold]

Fixes #269

[iliekturtles]

If you could also wrap the changes in lib.rs (diff below) I'll accept!

diff --git a/src/lib.rs b/src/lib.rs
index 22d7816..be430b4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -98,9 +98,9 @@
 //!  * `use_serde` -- Feature to enable support for serialization and deserialization of quantities
 //!    with the [Serde][serde] crate. Disabled by default.
 //!
-//!    [Serde][serde] support for the `big*` and `rational*` underlying storage types requires manually
-//!    enabling the `serde` feature for the `num-rational` and `num-bigint` crates. To do so, you can
-//!    add one or both of the following lines to your `Cargo.toml`:
+//!    [Serde][serde] support for the `big*` and `rational*` underlying storage types requires
+//!    manually enabling the `serde` feature for the `num-rational` and `num-bigint` crates. To do
+//!    so, you can add one or both of the following lines to your `Cargo.toml`:
 //!
 //!    ```toml
 //!    num-rational = { version = "*", features = ["serde"] }

[adamreichold]

If you could also wrap the changes in lib.rs (diff below)

Applied.

[iliekturtles]

Thanks for the PR. Sorry it took so long to get resolved.