Document checksum verification, mirrors, and caching

* `RUBY_BUILD_BUILD_PATH` sets the location in which sources are

  downloaded and built. By default, this is a subdirectory of

  `TMPDIR`.

* `RUBY_BUILD_CACHE_PATH`, if set, specifies a directory to use for

  caching downloaded package files.

* `RUBY_BUILD_MIRROR_URL` overrides the default mirror URL root to one

  of your choosing.

* `RUBY_BUILD_SKIP_MIRROR`, if set, forces ruby-build to download

  packages from their original source URLs instead of using a mirror.

* `CC` sets the path to the C compiler.

* `CONFIGURE_OPTS` lets you pass additional options to `./configure`.

* `MAKE_OPTS` (or `MAKEOPTS`) lets you pass additional options to

  `make`.

### Checksum verification

If you have the `md5`, `openssl`, or `md5sum` tool installed,

ruby-build will automatically verify the MD5 checksum of each

downloaded package before installing it.

Checksums are optional and specified as anchors on the package URL in

each definition. (All bundled definitions include checksums.)

### Package download mirrors

ruby-build will first attempt to download package files from a mirror

hosted on Amazon S3. If a package is not available on the mirror, if

the mirror is down, or if the download is corrupt, ruby-build will

fall back to the official URL specified in the defintion file.

You can point ruby-build to another mirror by specifying the

`RUBY_BUILD_MIRROR_URL` environment variable--useful if you'd like to

run your own local mirror, for example. Package mirror URLs are

constructed by joining this variable with the MD5 checksum of the

package file.

If you don't have an MD5 program installed, ruby-build will skip the

download mirror and use official URLs instead. You can force

ruby-build to bypass the mirror by setting the

`RUBY_BUILD_SKIP_MIRROR` environment variable.

### Package download caching

You can instruct ruby-build to keep a local cache of downloaded

package files by setting the `RUBY_BUILD_CACHE_PATH` environment

variable. When set, package files will be kept in this directory after

the first successful download and reused by subsequent invocations of

`ruby-build` and `rbenv install`.

The `rbenv install` command defaults this path to `~/.rbenv/cache`, so

in most cases you can enable download caching simply by creating that

directory.

### Keeping the build directory after installation

Both `ruby-build` and `rbenv install` accept the `-k` or `--keep`