Merge branch 'doc/release-cycle' into 'dev'

doc/releases.md: release cycle

Closes vdj#1173

See merge request !912

doc/releases.md

+
+# Release cycles
+
+The Vidjil project tries to keep [agile develompent](https://en.wikipedia.org/wiki/Agile_software_development) while producing certified, qualified healthcare software for clinical needs.
+
+- We target [continuous delivery](https://en.wikipedia.org/wiki/Continuous_delivery)
+  on the public server [app.vidjil.org](http://app.vidjil.org).
+  As of 2021, 
+  <ul>
+    <li> the client is intended to be semi-automatically deployed through Gitlab, 
+    <li> the server is manually deployed,
+    <li> the analysis engine [vidjil-algo](vidjil-algo.md), which follows his own release cycles, is manually deployed.
+  </ul> 
+  New releases with incremental changes may thus be deployed on-the-fly, but a notification is issued on the server.
+  However, we announce the most significant features impacting data analysis with a 2-week notice.
+  
+- The [healthcare servers](healthcare.md) get a major release twice a year, in June and December,
+  of all their components (client, server, analysis engine).
+  Any release on the healthcare server is intended to be in production for at least *one month* on the public server.
+  Such a release is further *qualified* with interaction with members of the [VidjilNet consortium](http://www.vidjil.net)
+  on their qualification datasets.
+
+Critical bug fixes can be deployed on both public and healthcare servers at any time.
+To help the development to move forward towards such stable healthcare releases,
+several [freeze](https://en.wikipedia.org/wiki/Freeze_(software_engineering)) windows are enforced.
+
+|  | Summer release ☀️ |  Winter release ❄️ |
+|--|--|--|
+| Meeting on release objectives | mid-February | begin-September |
+| *Dev: Main Freeze* <br /> Any new feature merge must be discussed. | 30 March | 31 September |
+| *Dev: Hard Freeze* <br /> No more feature merge, only bug fixes. <br /> Polish strings, documentation, release notes, and announcements. | 15 April | 15 October |
+| *Deploy on the public server* <br/>Continuous deployment on the public server is freezed, except for critical bug fixes. | before 30 April | before 30 October |
+| *Deploy stable release on healthcare servers and qualification* <br />Continuous deployment on the public servers resumes. |  June   |  December |
+
+
+For [server mainteners](server.md), the last stable release of the complete Vidjil platform is available on <https://hub.docker.com/u/vidjil>. 
+For bioinformaticians, the last stable release of the [vidjil-algo](vidjil-algo.md) analysis engine is available on <http://www.vidjil.org/releases/vidjil-latest.tar.gz>.
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