Add migration guide link for next release(blocked by release) (#555)

It's a part of task in
#364. This should not be
merged until v1.30.0 is released.

---------

Co-authored-by: Motoi Okuzono <motoi.okuzono+github@gmail.com>

Author: Yang-33

README.md

@@ -89,6 +89,9 @@ post '/callback' do
 end
 ```
 
+## Migration way from v1 to v2
+Migration guide: https://github.com/line/line-bot-sdk-ruby/releases/tag/v1.30.0
+
 ## Help and media
 FAQ: https://developers.line.biz/en/faq/
 

README_v2.md

@@ -297,6 +297,9 @@ See https://semver.org/
 v1 and v2 are completely different implementations, v1 is all deprecated and will be removed in the future.
 Migration to v2 is strongly recommended.
 
+Migration guide: https://github.com/line/line-bot-sdk-ruby/releases/tag/v1.30.0
+
+
 #### Deprecation warnings
 When calling the API endpoint with v1, a deprecation warning will occur.\
 If you want to suppress this, please set any value to the environment variable `SUPRESS_V1_DEPRECATION_WARNINGS`.