[incubator-skywalking] branch inventory-extend updated: Provide develop doc for inventory extension.

[wu...@apache.org]

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wusheng pushed a commit to branch inventory-extend
in repository https://gitbox.apache.org/repos/asf/incubator-skywalking.git


The following commit(s) were added to refs/heads/inventory-extend by this push:
     new abfede8  Provide develop doc for inventory extension.
abfede8 is described below

commit abfede8a51952767921b275d76300eccc243b115
Author: Wu Sheng <wu...@foxmail.com>
AuthorDate: Fri Jan 11 20:30:42 2019 +0800

    Provide develop doc for inventory extension.
---
 docs/en/guides/README.md              |  2 ++
 docs/en/guides/inventory-extension.md | 31 +++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/docs/en/guides/README.md b/docs/en/guides/README.md
index 26f0033..af3c192 100644
--- a/docs/en/guides/README.md
+++ b/docs/en/guides/README.md
@@ -37,6 +37,8 @@ and private plugin developer should read this.
 - [Storage extension development guide](storage-extention.md). Help potential contributors to build a new 
 storage implementor besides the official.
 - [Customize analysis by oal script](write-oal.md). Guide you to use oal script to make your own metric available.
+- [Backend Inventory entity extension](inventory-extension.md). If you want to extend SkyWalking inventory entities, and
+want to push upstream back to our Apache OSS repo, please read these principles.
 
 ### UI developer
 Our UI is constituted by static pages and web container.
diff --git a/docs/en/guides/inventory-extension.md b/docs/en/guides/inventory-extension.md
new file mode 100644
index 0000000..cee70eb
--- /dev/null
+++ b/docs/en/guides/inventory-extension.md
@@ -0,0 +1,31 @@
+# Backend Inventory Entity Extension
+SkyWalking includes four inventory entities.
+- Service Inventory
+- Service Instance Inventory
+- Endpoint Inventory
+- Network Address Inventory
+
+All metric, topology, trace and alarm are related to these entity IDs. 
+
+For understanding the **Service**, **Service Instance** and **Endpoint** concepts,
+please read [Project Overview](../concepts-and-designs/overview.md#why-use-skywalking).
+
+For **Network Address Inventory**, it represents all network address, in IP:port, hostname, domain name
+formats, which are detected by language agents or other probes.
+
+## Extension
+Right now, only **Service Inventory** extension is already supported in backend core.
+Service provides field `properties` in Json format, which is usually used for specific service 
+rather than normal business services, such as Database, Cache, MQ, etc.
+
+For keeping code consistent and friendly in query and visualization, the Json properties
+need to follow the rules.
+
+### Database
+1. NodeType == **Database(1)**
+1. Json properties include following keys.
+  - `database`. Database name, such as MySQL, PostgreSQL
+  - `db.type`. Database type, such as sql db, redis db.
+  - `db.instance`. Database instance name.
+
+