docs: Added warning about existing tag in semver (argoproj-labs#471)

Signed-off-by: Kostis Kapelonis <kostis@codefresh.io>

Signed-off-by: Kostis Kapelonis <kostis@codefresh.io>

docs/basics/update-strategies.md

@@ -77,6 +77,10 @@ The above example would update to any new tag pushed to the registry matching
 this constraint, e.g. `1.2.5`, `1.2.12` etc, but not to a new minor version
 (e.g. `1.3`).
 
+!!!warning "A note on the current image tag"
+    For semver strategy to work, the current application tag must already follow
+    semver. Otherwise no comparison can happen by the updater. See discussion at [#270](https://github.com/argoproj-labs/argocd-image-updater/issues/270) for more details.
+
 Likewise, to allow updates to any minor release within the major version `1`,
 use
 
@@ -101,6 +105,8 @@ Image Updater will pick the highest version number found in the registry.
 Argo CD Image Updater will omit any tags from your registry that do not match 
 a semantic version when using the `semver` update strategy.
 
+
+
 ### <a name="strategy-latest"></a>latest/newest-build - Update to the most recently built image