Add changelog and upgrading info for v0.13.0

MSP-11245
MSP-11247
MSP-11248

CHANGELOG.md

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+# Changelog
+
+## v0.13.0
+
+* Enhancements
+  * [#76](https://github.com/rapid7/metasploit-credential/pull/76) - [@dmaloney](https://github.com/dmaloney-r7)
+    * Blank passwords (`Metasploit::Credential::BlankPassword`) and usernames (`Metasploit::Credential::BlankUsername`),
+      including importer detecting `<BLANK>`.
+    * Metasploit::Credential::Core` with only a realm is now valid.
+
+* Incompatible Changes
+  * [#76](https://github.com/rapid7/metasploit-credential/pull/76) - [@dmaloney](https://github.com/dmaloney-r7)
+    * `Metasploit::Credential::Public` uses Single-Table Inheritance (STI), so SQL access need to be aware of the `type` column.
+    * The `:metasploit_credential_public` factory will now randomly generate a `Metasploit::Credential::BlankUsername`
+      or `Metasploit::Credential::Username`.  If you use `:metasploit_credential_public`, you need to ensure that you
+      can handle blank usernames, otherwise use `:metasploit_credential_username` to get the old behavior.

UPGRADING.md

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+# Upgrading
+
+## V0.13.0
+
+* If you use `:metasploit_credential_public`, you need to ensure that you can handle blank usernames, otherwise use
+  `:metasploit_credential_username` to get the old behavior.