Better clarifications for publishing snaps (MetaMask#897)

* Update snaps publishing documentation - fixes MetaMask#643

1. Clarify the requirements for the icon
2. Call out that only the snap package should be published when using the template.
3. Mention how to use the Simulator to test it.

* Update develop-a-snap.md

Clarify all surfaces for snap icon

snaps/how-to/develop-a-snap.md

@@ -86,11 +86,19 @@ on publishing to the public registry.
 The following details are specific to Snaps:
 
 - The version in `package.json` and `snap.manifest.json` must match.
-- The image specified in `iconPath` in the manifest file is used as the icon displayed when
-  installing and displaying confirmations from the snap.
+- The image specified in `iconPath` in the manifest file is used as the icon displayed when installing the snap, in custom dialogs, and in the settings menu.
+  - This icon must be a valid SVG.
+  - The icon will be cropped in a circle when displayed in MetaMask; you do not need to make the icon circular.
 
 After publishing the snap, any dapp can connect to the snap by using the snap ID `npm:[packageName]`.
 
+:::caution
+If you are using the snap template, make sure to only publish the snap package in `/packages/snap`. 
+You can use the [Snaps Simulator](https://metamask.github.io/snaps/snaps-simulator/staging/#/manifest) to verify 
+that your snap was published correctly — just click on "localhost" in the top right corner and change the 
+snap location to be "npm" and the ID of your snap. 
+:::
+
 ## Distribute your snap
 
 Since snaps are currently intended for a developer audience, MetaMask doesn't currently facilitate