[doc] add developer guide which contains release process tutorial

doc/developer-guide.md

+# Developer guide
+
+### Releases
+To do releases, there is a script which handle it for us.
+All services included GitHub will automatically create a release.
+
+#### Pre-releases
+```bash
+./release.sh pre 0.40.0a4
+```
+
+#### Stable releases
+```bash
+./release.sh rel 0.40.0
+```
+
+Releases are based on tags. This script will tag the commit.
+
+So, you will have to push tags to trigger releases:
+```bash
+git push --tags
+```