readme: add badges

crates.io used to show some automatically per badge configuration in the
crate toml files, however then they dropped this ([1]). crates now show
badges via embedding them in readme.md files.

shields.io is a popular service for constructing and serving badges, with
a lot of available customisation.

here i have selected the 'for-the-badge' style, which is similar to the
nice 'flat-square' style, but uses capital letters and a default later
height of 28px instead of 20px. I have used HTML for embedding them to
allow control over the size, which I have forced to 24px since 28px is just
too large.

notes:
 - for the min-rust-version badge, there were two different URL formats
   available for custom badges, I went with the 'static' form which makes
   use of a lot of query params.
 - for the min-rust-version badge, I had to use capitals for the service
   text to get it consistent with the rest, since it was not automatically
   capitalising it.
 - for the docs.rs one i had to use the crates.io one with a custom colour
   and service text specified, since shields.io has a mechanism in place
   for automatically building the latest crates.io version number into the
   crates.io badge, but does not have a similar specific badge-builder
   available for docs.rs badges. (and the badge you can get from docs.rs
   itself is a different style of course).
 - the message for the min-rust-version is '1.40%2B' which is '1.40+'!

i was inspired by the badges of the 'quote' crate.

[1]: rust-lang/crates.io#2436

Author: jnqnfe

README.md

@@ -1,6 +1,9 @@
 Overview
 ========
 
+[<img alt="travis.com" src="https://img.shields.io/travis/com/jnqnfe/gong?style=for-the-badge" height="24">](https://travis-ci.com/jnqnfe/gong)
+<img alt="license" src="https://img.shields.io/crates/l/gong.svg?style=for-the-badge" height="24">
+
 This repository contains `gong`, a lightweight, flexible and simple-to-use library provided to
 assist in processing command line arguments in Rust based programs.
 

lib/README.md

@@ -1,6 +1,11 @@
 gong
 ====
 
+[<img alt="travis.com" src="https://img.shields.io/travis/com/jnqnfe/gong?style=for-the-badge" height="24">](https://travis-ci.com/github/jnqnfe/gong)
+[<img alt="crates.io" src="https://img.shields.io/crates/v/gong?style=for-the-badge" height="24">](https://crates.io/crates/gong)
+[<img alt="docs.rs" src="https://img.shields.io/crates/v/gong?color=5479ab&label=docs.rs&style=for-the-badge" height="24">](https://docs.rs/gong)
+[<img alt="min-rust-version" src="https://img.shields.io/static/v1?label=RUST&message=1.26%2B&color=informational&style=for-the-badge" height="24">](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)
+
 A lightweight, flexible and simple-to-use library provided to assist in processing command line
 arguments.