Cleanup wrapping, whitespace, etc. in README.md

In `README.md`, some lines were hard wrapped where others were not,
there was some trailing whitespace, and some list items were
terminated with a period where some were not.  Let's clean these
things up by hard wrapping all lines, cleaning up the whitespace, and
terminating all list items with periods.

Author: traviscross

README.md

@@ -1,6 +1,7 @@
 # The Rust Language Reference
 
-This document is the primary reference for the Rust programming language.
+This document is the primary reference for the Rust programming
+language.
 
 This document is not normative. It may include details that are specific
 to `rustc` itself, and should not be taken as a specification for the
@@ -10,15 +11,18 @@ what we have for now.
 ## Dependencies
 
 - rustc (the Rust compiler).
-- [mdbook](https://rust-lang.github.io/mdBook/) (use `cargo install mdbook` to install it).
-- rust nightly (you would be required to set your Rust version to the nightly version to make sure all tests pass)
+- [mdbook](https://rust-lang.github.io/mdBook/) (use `cargo install
+  mdbook` to install it).
+- rust nightly (you would be required to set your Rust version to the
+  nightly version to make sure all tests pass).
 
 ## Build steps
 
-To build the project, follow the steps given below :
+To build the project, follow the steps given below:
 
-Clone the project by downloading the ZIP from the [GitHub page](https://github.com/rust-lang/reference) or
-run the following command:
+Clone the project by downloading the ZIP from the [GitHub
+page](https://github.com/rust-lang/reference) or run the following
+command:
 
 ```
 git clone https://github.com/rust-lang/reference
@@ -30,21 +34,24 @@ Change the directory to the downloaded repository:
 cd reference
 ```
 
-To run the tests, you would need to set the Rust version to the nightly release. You can do this by executing the following command:
+To run the tests, you would need to set the Rust version to the nightly
+release. You can do this by executing the following command:
 
 ```shell
 rustup override set nightly
 ```
 
 This will set the nightly version only for your the current project.
 
-If you wish to set Rust nightly for all your projects, you can run the command: 
+If you wish to set Rust nightly for all your projects, you can run the
+command:
 
 ```shell
 rustup default nightly
 ```
 
-Now, run the following command to test the code snippets to catch compilation errors:
+Now, run the following command to test the code snippets to catch
+compilation errors:
 
 ```shell
 mdbook test