Miscellaneous README changes

[iKevinY]

• Various grammatical changes.
• Use triple-backtick syntax and sh highlighting for code blocks.
• Fix indentation of code block in step 2 of "Building on Windows".
• Use title case for "Getting Help" subheading.

[rust-highfive]

r? @brson

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[iKevinY]

Also, here's a rendered version. r? @steveklabnik

[steveklabnik]

if we're using triple graves, then we shouldn't need to indent

[steveklabnik]

r= me after a few nits, thanks!

[iKevinY]

@steveklabnik I think the indentation is still necessary in order to keep the code blocks / blockquotes on the same "level" as the ordered lists (unless the content after the code block in step 3 of "Building From Source" should be excluded from the list). The list numberings get messed up if a top-level element appears in the middle of list (see the "Building on Windows" list on the current version of the README).

[steveklabnik]

Ahhhhh

[steveklabnik]

@bors: r+ 24fa6be rollup

[iKevinY]

@steveklabnik That being said, maybe the paragraph after the note in "Building from Source" should be placed at the top level, since it's just information rather than a "step" to follow; thoughts on that change?