doc: Add maintainer process

This patch adds basic maintainer process description, that has been
used so far since the project start.

Signed-off-by: Alexander Smirnov <asmirnov@ilbers.de>

doc/maintainer.md

@@ -0,0 +1,56 @@
+# Introduction
+
+Isar maintainer is the person who keeps the project development in
+compliance with the contributing process definition. The purpose of
+this document is to describe the maintainer process in the Isar
+project.
+
+# Responsibilities
+
+## Feedback on Contributions
+
+The Isar maintainer should review each contribution that has been sent
+to the mailing list and provide feedback. The feedback could be on the
+following categories:
+
+ - Contribution accepted: If no quality and design issues were found
+   by the maintainer and the other mailing list members.
+
+ - Change request: If some small quality or design issues were found,
+   the maintainer or the other mailing list memeber requests a new
+   version with resolved issues.
+
+ - Contribution rejected: If there are major design issues or general
+   benefit from the contribution is ambiguous.
+
+During the review, the maintainer may ask additional questions to
+clarify the details. Any other mailing list member could assist the
+maintainer in the review process.
+
+## Repository Branches
+
+There are two official branches in Isar, intended to increase project
+quality:
+
+ - next: The accepted contributions from the mailing list are merged
+   to this branch. Basic CI checks should be run after each merge.
+   This branch could be changed non-linearly.
+
+ - master: Is the official stable branch. The only way patches go in
+   here is coming from 'next', where they have passed all required
+   tests and undergone the review process. On this branch force-pushing
+   will never be used.
+
+The next branch is intended to be merged into master monthly. The
+maintainer may perform a merge to master before the ordinary window
+if the next branch contains urgent patch series (bug fixes or critical
+features).
+
+## Issues at GitHub
+
+For each issue that has been found in master and next branches the
+maintainer should create a GitHub issue. The issue should reflect to
+the one of the following:
+ - Bug in the existing Isar code.
+ - New Isar feature that is planned to be developed.
+ - Improvement that would be nice to have in Isar.