Update README.md (#4175)

* update doc version

* ref 6.x branch

* ref 6.x branch

* update doc/README.md

* remove 5.x ...

README.md

@@ -44,7 +44,9 @@ including
 
 
 # Document
-- [6.x Documents](docs/README.md).
+- [7.x Documents](docs/README.md).
+- [6.x Documents](https://github.com/apache/skywalking/blob/6.x/docs/README.md).
+
 
 
 # Downloads

docs/README.md

 # Welcome
-**Here are SkyWalking 6 official documents. You're welcome to join us.**
+**Here are SkyWalking 7 official documents. You're welcome to join us.**
 
 From here you can learn all about **SkyWalking**’s architecture, how to deploy and use SkyWalking, and develop based on SkyWalking contributions guidelines.
 
@@ -91,14 +91,3 @@ If you are already familiar with SkyWalking, you could use this catalog to find
 * [FAQs](en/FAQ/README.md). A manifest of already known setup problems, secondary developments experiments. When you are facing a problem, check here first.
 
 
-___
-### Users from 5.x
-
-SkyWalking 5.x is still supported by the community. For the users planning to upgrade from 5.x to 6.x, you should know some definitions of concepts changed.
-
-The most important two changed concepts are:
-
-1. Application (in 5.x) is changed to **Service** (in 6.x), also Application Instance is changed to **Service Instance**.
-2. Service (in 5.x) is changed to **Endpoint** (in 6.x).
-
-