Add UPGRADING.md

UPGRADING.md

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+# From v14 to v15
+- Upgrade PHP to 8.0
+- Execute `php artisan bjyblog:update` command
+
+# From v6 to v14
+- Upgrade PHP to 7.3
+- Execute `php artisan bjyblog:update` command
+
+# From v5.8 to v6
+- Upgrade PHP to 7.2
+- Execute `php artisan bjyblog:update` command
+
+# From v5.5 to v5.8
+- Upgrade PHP to 7.1
+- Execute `php artisan bjyblog:update` command
+
+# From v1 to v5.5
+- Upgrade PHP to 7.0
+- Change the type of the `markdown` and `html` column of table `articles` from `text` to `mediumtext`
+- Execute `php artisan bjyblog:update` command