Update release notes.

We need to set the assets path for CDN builds

RELEASE.md

 # Release checklist
 
 1. Run `make check` to check that all tests pass.
-2. Decide on a version number, e.g. 5.0.0
-3. Run `make release VERSION=5.0.0`
+2. Decide on a version number, e.g. 6.0.0
+3. Run `make release VERSION=6.0.0`
 4. Do a `git diff` to check if things look sane.
 5. Do a quick manual test with the `dist` files (via `index.html`)
-6. `git commit -am "Release 5.0.0"`
-7. `git tag -s v5.0.1 m "Release 5.0.0"`
+6. `git commit -am "Release 6.0.0"`
+7. `git tag -s v6.0.0 m "Release 6.0.0"`
 8. Run `git push && git push --tags`
 9. Update https://conversejs.org
-10. Create `5.0.0` directory for the CDN.
-    * Create a new version for the CDN by copying
-    * Check out the correct tag
-    * Update `index.html` to point to that version of the CDN
-    * Run `make dist`
-    * Do the same for the root dir
+10. Create `6.0.0` directory for the CDN.
+    * `cd /home/conversejs/converse.js`
+    * `git clone --branch v6.0.0 git@github.com:conversejs/converse.js.git 6.0.0`
+    * `cd 6.0.0 && ASSET_PATH=https://cdn.conversejs.org/6.0.0/dist/ make dist`
+    * `cd .. && git pull && ASSET_PATH=https://cdn.conversejs.org/dist/ make dist`
 11. Run `npm publish && cd src/headless/ && npm publish`
 12. Update the repository on weblate
-13. Decide on next release number and run `make postrelease VERSION=5.0.2`
+13. Decide on next release number and run `make postrelease VERSION=6.0.1`