Merge pull request #1348 from ross-bragg/improve-ruby-build-mirror-example

| `RUBY_BUILD_CURL_OPTS`   | Additional options to pass to `curl` for downloading.                                            |

| `RUBY_BUILD_WGET_OPTS`   | Additional options to pass to `wget` for downloading.                                            |

| `RUBY_BUILD_MIRROR_URL`  | Custom mirror URL root.                                                                          |

| `RUBY_BUILD_SKIP_MIRROR` | Always download from official sources, not mirrors. (Default: unset)                             |

| `RUBY_BUILD_SKIP_MIRROR` | Bypass the download mirror and fetch all package files from their original URLs.                  |

| `RUBY_BUILD_ROOT`        | Custom build definition directory. (Default: `share/ruby-build`)                                 |

| `RUBY_BUILD_DEFINITIONS` | Additional paths to search for build definitions. (Colon-separated list)                         |

| `CC`                     | Path to the C compiler.                                                                          |

installing it.

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

definition. (All bundled definitions include checksums.)

definition. All definitions bundled with ruby-build include checksums.

#### Package Mirrors

By default, ruby-build downloads package files from a mirror hosted on Amazon

CloudFront. 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 definition file.

To speed up downloads, ruby-build fetches package files from a mirror hosted on

Amazon CloudFront. To benefit from this, the packages must specify their checksum:

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 SHA2 checksum of the package file.

```sh

# example:

install_package "ruby-2.6.5" "https://ruby-lang.org/ruby-2.6.5.tgz#<SHA2>"

```

ruby-build will first try to fetch this package from `$RUBY_BUILD_MIRROR_URL/<SHA2>`

(note: this is the complete URL), where `<SHA2>` is the checksum for the file. It

will fall back to downloading the package from the original location if:

- the package was not found on the mirror;

- the mirror is down;

- the download is corrupt, i.e. the file's checksum doesn't match;

- no tool is available to calculate the checksum; or

- `RUBY_BUILD_SKIP_MIRROR` is enabled.

If you don't have an SHA2 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.

You may specify a custom mirror by setting `RUBY_BUILD_MIRROR_URL`.

The official ruby-build download mirror is sponsored by

The default ruby-build download mirror is sponsored by

[Basecamp](https://basecamp.com/).

#### Keeping the build directory after installation