Use HTTPS URL for the submodule (#17)

- Fix RELEASE_NOTES template
- Clear release notes
- Use HTTPS to retrieve the submodule

Author: llucax

.github/RELEASE_NOTES.template.md

@@ -1,4 +1,4 @@
-# Frequenz Migrogrid API Release Notes
+# Frequenz Common API Release Notes
 
 ## Summary
 

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "submodules/api-common-protos"]
 	path = submodules/api-common-protos
-	url = git@github.com:googleapis/api-common-protos.git
+	url = https://github.com/googleapis/api-common-protos.git

RELEASE_NOTES.md

@@ -2,5 +2,23 @@
 
 ## Summary
 
-- Various build system fixes
-- Improved smoothness of using this as a dependency
+<!-- Here goes a general summary of what this release is about -->
+
+## Upgrading
+
+* The submodule URL was changed to use HTTPS instead of SSH (to avoid problems trying to unlock SSH keys to do updates, etc.).
+
+  Make sure you sync your submodules to the new URL:
+
+  ```console
+  $ git submodule sync
+  Synchronizing submodule url for 'submodules/api-common-protos'
+  ```
+
+## New Features
+
+<!-- Here goes the main new features and examples or instructions on how to use them -->
+
+## Bug Fixes
+
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->