Every release gets a number.
Semantic versioning uses three numbers: major, minor and patch. Anyone can see how big a change is from the number alone.
1.0.0
First release
1.0.1
Patch: a bug fixed
1.1.0
Minor: a feature added
2.0.0
Major: something changed that could break
With every release
Bump the number.
Tag it in Git.
Add a line to the changelog.
Show it in the app's footer.
Source: Semantic Versioning 2.0.0 ↗
Number it. Note it. Tag it.