Add macos specific upgrade instructions

Add macos specific upgrade instructions to the update notification that
way it is easier for people to just copy and paste the commmands and run
them rather than have to go look them up on the website.

This resolves issue #156.

[changelog]
added: macos specific update instructions in update notification

<!-- ps-id: ad1bd0c9-822f-49e6-8208-ad8184b726ab -->

src/ps/public/latest_github_release.rs

@@ -92,6 +92,7 @@ pub fn newer_release_available() -> Result<Option<GitHubRelease>, NewerReleaseAv
     }
 }
 
+#[cfg(not(target_os = "macos"))]
 pub fn notify_of_newer_release(newer_release: Option<GitHubRelease>, color: bool) {
     if let Some(latest_release) = newer_release {
         utils::print_warn(
@@ -108,3 +109,23 @@ pub fn notify_of_newer_release(newer_release: Option<GitHubRelease>, color: bool
         )
     }
 }
+
+#[cfg(target_os = "macos")]
+pub fn notify_of_newer_release(newer_release: Option<GitHubRelease>, color: bool) {
+    if let Some(latest_release) = newer_release {
+        utils::print_warn(
+            color,
+            format!(
+                r#"
+  A new release of gps is available!
+
+  {} - {}
+
+  To upgrade, run: brew update && brew upgrade git-ps-rs
+"#,
+                latest_release.tag_name, latest_release.html_url
+            )
+            .as_str(),
+        )
+    }
+}