Cherry-pick #9767 to 6.6: Introduce next Changelog file (#9808)

Cherry-pick of PR #9767 to 6.6 branch. Original message: 

This introduces CHANGELOG.next.asciidoc and
CHANGELOG-developer.next.asciidoc. These changelog files will document
yet unreleased changes.

Having a separate changelog file reduces the work required to cleanup
the changelog, as rebasing and cherry-picking on backports sometimes
moves changelog entries into the wrong place.

I don't mean all PRs to be updated/changed to use the CHANGELOG.next.asciidoc yet. We will continue cleaning up the changelog, until things eventually get more stabilisied by everyone using CHANGELOG.next.asciidoc in the future. In order to reduce some friction for other PRs we still have the current changelog entries in CHANGELOG.asciidoc. We will remove the template from CHANGELOG.asciidoc during the next releases.

Note: the developers changelog requires quite some cleanup. Switching to CHANGELOG-developer.next.asciidoc will make our lives easier here as well.

CHANGELOG-developer.asciidoc

@@ -13,9 +13,9 @@ other Beats should be migrated.
 Note: This changelog was only started after the 6.3 release.
 
 === Beats version HEAD
-https://github.com/elastic/beats/compare/v6.3.0..master[Check the HEAD diff]
+https://github.com/elastic/beats/compare/v6.3.0..v7.0.0-alpha2[Check the HEAD diff]
 
-The list below covers the major changes between 6.3.0 and master only.
+The list below covers the major changes between 6.3.0 and 7.0.0-alpha2 only.
 
 ==== Breaking changes
 

CHANGELOG-developer.next.asciidoc

@@ -0,0 +1,25 @@
+// Use these for links to issue and pulls. Note issues and pulls redirect one to
+// each other on Github, so don't worry too much on using the right prefix.
+:issue: https://github.com/elastic/beats/issues/
+:pull: https://github.com/elastic/beats/pull/
+
+This changelog is intended for community Beat developers. It covers the major
+breaking changes to the internal APIs in the official Beats and changes related
+to developing a Beat like code generators or `fields.yml`. Only the major
+changes will be covered in this changelog that are expected to affect community
+developers. Each breaking change added here should have an explanation on how
+other Beats should be migrated.
+
+Note: This changelog documents the current changes which are not yet present in
+an actual release.
+
+=== Beats version HEAD
+https://github.com/elastic/beats/compare/v6.6.0..6.6[Check the HEAD diff]
+
+The list below covers the major changes between 6.6.0 and 6.6 head only.
+
+==== Breaking changes
+
+==== Bugfixes
+
+==== Added

CHANGELOG.next.asciidoc

@@ -0,0 +1,91 @@
+// Use these for links to issue and pulls. Note issues and pulls redirect one to
+// each other on Github, so don't worry too much on using the right prefix.
+:issue: https://github.com/elastic/beats/issues/
+:pull: https://github.com/elastic/beats/pull/
+
+=== Beats version HEAD
+https://github.com/elastic/beats/compare/1035569addc4a3b29ffa14f8a08c27c1ace16ef9...6.6[Check the HEAD diff]
+
+==== Breaking changes
+
+*Affecting all Beats*
+
+*Auditbeat*
+
+*Filebeat*
+
+*Heartbeat*
+
+*Journalbeat*
+
+*Metricbeat*
+
+*Packetbeat*
+
+*Packetbeat*
+
+*Winlogbeat*
+
+*Functionbeat*
+
+==== Bugfixes
+
+*Affecting all Beats*
+
+*Auditbeat*
+
+*Filebeat*
+
+*Heartbeat*
+
+*Journalbeat*
+
+*Metricbeat*
+
+*Packetbeat*
+
+*Winlogbeat*
+
+*Functionbeat*
+
+==== Added
+
+*Affecting all Beats*
+
+- Release Jolokia autodiscover as GA. {pull}9706[9706]
+
+*Auditbeat*
+
+*Filebeat*
+
+*Heartbeat*
+
+*Journalbeat*
+
+*Metricbeat*
+
+*Packetbeat*
+
+*Winlogbeat*
+
+*Functionbeat*
+
+==== Deprecated
+
+*Affecting all Beats*
+
+*Filebeat*
+
+*Heartbeat*
+
+*Journalbeat*
+
+*Metricbeat*
+
+*Packetbeat*
+
+*Winlogbeat*
+
+*Functionbeat*
+
+==== Known Issue

docs/devguide/contributing.asciidoc

@@ -27,7 +27,7 @@ You only need to sign the CLA once.
 https://help.github.com/articles/using-pull-requests[submit a pull request]. In
 the pull request, describe what your changes do and mention any bugs/issues
 related to the pull request. Please also add a changelog entry to
-https://github.com/elastic/beats/blob/master/CHANGELOG.asciidoc[CHANGELOG.asciidoc].
+https://github.com/elastic/beats/blob/master/CHANGELOG.next.asciidoc[CHANGELOG.next.asciidoc].
 
 [float]
 [[adding-new-beat]]
@@ -147,4 +147,4 @@ In most cases `govendor fetch your/dependency@version +out` will get the job don
 
 To keep up to date with changes to the official Beats for community developers,
 follow the developer changelog
-https://github.com/elastic/beats/blob/master/CHANGELOG-developer.asciidoc[here].
+https://github.com/elastic/beats/blob/master/CHANGELOG-developer.next.asciidoc[here].