Update README.md

Author: takezoe

README.md

@@ -18,6 +18,13 @@ Plugin version | GitBucket version
 
 Download jar file from [the release page](https://github.com/gitbucket/gitbucket-gist-plugin/releases) and put into `GITBUCKET_HOME/plugins`.
 
+**Note:** If you had used this plugin with GitBucket 3.x, it does not work after upgrading GitBucket 4.x. Solution is below:
+
+1. `UPDATE VERSIONS SET VERSION='2.0.0' WHERE MODULE_ID='gist';`
+2. restart gitbucket
+3. can open snippets page
+4. `SELECT VERSION FROM VERSIONS WHERE MODULE_ID='gist'` -> `4.2.0`
+
 ## Build from source
 
 1. Hit `./sbt.sh package` in the root directory of this repository.