add documentation for validate-downloads job

jenkins/jobs.md

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+# Jenkins Jobs
+
+## [validate-downloads](https://ci.nodejs.org/view/All/job/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 60 minutes) so releasers may have to wait a bit
+be able to complete the validation.
+
+If validation fails an email notification is sent to the
+`release-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.