add documentation for validate-downloads job

jenkins/doc/jobs/validate-downloads.md

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+# validate-downloads
+
+This job validates that the downloads on nodejs.org are good. 
+It is scheduled to be run nightly and can also be run manually
+after a release is made. **Note:** there is delay between
+when releases are generated and when they will be available on
+nodejs.org(up to 15 minutes) so releasers may have to wait
+a bit be able to complete the validation.
+
+It has the following build parameters:
+
+* `StreamAndVersion`: This can either be one of `latest`, `release`, `vX`,
+  or a fully qualified version (ex v8.4.0). If you select `release` then the
+  latest release for all of the active LTS releases and the Current release
+  will be validated. If you select `latest` then the
+  latest release for all of the active LTS releases, the Current release
+  as well as the nightlies will be validated.
+  If you select 'vX' where X is is for a stream (ex v8, v7, v4) then the
+  latest release from that stream will be validated.
+  If you select a fully qualified version (ex v8.4.0) then that specific
+  release will be validated.
+* `DownloadLocation`: This controls whether the release or nightly download
+  location is used. For example f you want to validate the latest v8 nightly
+  you would set StreamAndVersion to 'v8' and DownloadLocation to 'nightly'.
+
+If validation fails an email notification is sent to the
+`download-validation-alert` email alias. If you would like to get these
+notifications submit a PR to have your email added for that alias in
+https://github.com/nodejs/email/blob/master/iojs.org/aliases.json.
+
+This job needs to be updated each time a we roll over to a new Current
+release.
+