[md] README: add note about status, link to current documentation
[md] README: add note about status, link to current documentation

artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,develar/intellij-sdk-docs,artspb/intellij-sdk-docs,chashnikov/intellij-sdk-docs,breandan/intellij-sdk-docs,artspb/intellij-sdk-docs,chashnikov/intellij-sdk-docs,JetBrains/intellij-sdk-docs,develar/intellij-sdk-docs,breandan/intellij-sdk-docs,JetBrains/intellij-sdk-docs,chashnikov/intellij-sdk-docs,develar/intellij-sdk-docs,chashnikov/intellij-sdk-docs,breandan/intellij-sdk-docs,JetBrains/intellij-sdk-docs