Clarify the core dependencies mappings: bbolt and raft

Signed-off-by: Benjamin Wang <[email protected]>

Documentation/contributor-guide/dependency_management.md

@@ -12,6 +12,7 @@
   - [Rotation worksheet](#rotation-worksheet)
 - **[Stable branches](#stable-branches)**
 - **[Golang versions](#golang-versions)**
+- **[Core dependencies mappings](#core-dependencies-mappings)**
 
 ## Main branch
 
@@ -126,3 +127,20 @@ For an example of how to update etcd to a new patch release of Go refer to issue
 References:
 
 - <https://github.com/kubernetes/sig-release/blob/master/release-engineering/handbooks/go.md>
+
+## Core dependencies mappings
+[bbolt](https://github.com/etcd-io/bbolt) and [raft](https://github.com/etcd-io/raft) are two core dependencies of etcd.
+
+Both etcd 3.4.x and 3.5.x depend on bbolt 1.3.x, and etcd 3.6.x (`main` branch) depends on bbolt 1.4.x.
+
+raft is included in etcd repository for release-3.4 and release-3.5 branches, so etcd 3.4.x and 3.5.x do not depend on any
+external raft module. We moved raft into [a separate repo](https://github.com/etcd-io/raft) starting from 3.6 (`main` branch), and the first raft
+release will be v3.6.0, so etcd 3.6.x will depend on raft 3.6.x.
+
+Please see the table below,
+
+| etcd versions | bbolt versions | raft versions |
+|---------------|----------------|---------------|
+| 3.4.x         | v1.3.x         | N/A           |
+| 3.5.x         | v1.3.x         | N/A           |
+| 3.6.x         | v1.4.x         | v3.6.x        |