Add an upgrade message for pre-release testers

Author: michaelblyons

messages.json

@@ -1,3 +1,4 @@
 {
+	"5.0.0-alpha.01": "messages/5.0.0-alpha.md",
 	"3.1.1": "messages/prerelease-invitation.md"
 }

messages/5.0.0-alpha.md

@@ -0,0 +1,43 @@
+# 5.0.0 Alpha
+
+Hello PowerShell user. Welcome to the 5.0 Alpha!
+
+If you find bugs, especially egregious ones that break code
+highlighting further down in the file, please mention them at
+https://github.com/SublimeText/PowerShell/issues and include "v5"
+in the title.
+
+Thank you.
+
+
+## Standout Improvements
+
+- Commands not in Verb-Noun format are now recognized.
+
+- Command arguments are more conscientiously marked,
+  including unquoted strings.
+
+- String contents are sometimes interpreted specially,
+  though not in all cases. Some examples:
+
+    * Here-strings beginning with `using System` are
+      highlighted with C#.
+
+    * Command argument strings recognize wildcards and
+      path separators.
+
+    * Single-quoted strings in certain cases are
+      highlighted with Regular Expressions.
+
+    * Other strings recognize simple format placeholders.
+
+- `using` imports can only come at the beginning of a file.
+
+- `meta` scopes are used in more places, which doesn't
+  typically affect highlighting, but sometimes has an
+  effect on ST commands.
+
+
+## Known Issues
+
+- #213 The Containerfile package exceeds the context sanity check.