Collector 5.0.0-alpha version upgrade FAQ. (#1052)

* Collector 5.0.0-alpha version upgrade FAQ.

* Update Upgrade.md

Reformat document and adjust some English.

* Add link.

docs/README.md

@@ -38,4 +38,5 @@
     * [Why only traces in UI?](cn/FAQ/Why-have-traces-no-others.md)
     * [Too many GRPC logs in the console](cn/FAQ/Too-many-gRPC-logs.md)
     * [The trace doesn't continue in kafka consumer side](en/FAQ/kafka-plugin.md)
+    * [Agent or collector version upgrade](en/FAQ/Upgrade.md)
     
\ No newline at end of file

docs/en/FAQ/Upgrade.md

+# Upgrade FAQs
+## Version 3.x -> 5.0.0-alpha
+### Collector
+- **Problem**
+There is no information showing in the UI.
+
+- **Cause**
+In upgrate from 3.2.6 to 5.0.0, Elasticsearch indexes aren't recreated, because not indexes exist, but aren't compatible with 5.0.0-alpha.
+When service name registered, the es will create this column by default type string, which is wrong.
+
+- **Solution**
+Clean the data folder in ElasticSearch and restart ElasticSearch, collector and your under monitoring application.