Create README for RT-7.6: link bandwidth aggregation. (#3098)

* Create README for RT-7.6: link bandwidth aggregation.

feature/bgp/linkbandwidth_community/otg_tests/linkbandwidth_aggregation/README.md

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+# RT-7.6: BGP Link Bandwidth Community - Cumulative
+
+## Summary
+
+This test verifies Link-bandwidth (LBW) extended community cumulative feature by DUT.
+
+## Testbed type
+
+*  [`featureprofiles/topologies/atedut_2.testbed`](https://github.com/openconfig/featureprofiles/blob/main/topologies/atedut_2.testbed)
+
+## Procedure
+
+### Test environment setup
+
+    ```
+                                          |         |
+    [ 64x eBGP ] --- [ ATE Port 1 ] ----  |   DUT   | ---- [ ATE Port 2 ]
+                                          |         |
+    ```
+
+#### Configuration
+
+* Configure DUT with 2 routed ports.
+* Configure 64x eBGP peers on ATE Port 1 interface
+* Configure 64x eBGP peers on DUT Port 1 interface in peering group UPSTREAM.
+* Configure a single eBGP peer on ATE Port 2 interface.
+* Configure a single eBGP peer on DUT Port 2 interface in peering group DOWNSTREAM.
+
+
+### RT-7.6.1: Verify LBW cumulative to eBGP peer
+
+* Enable BGP LBW receive for peering group UPSTREAM.
+* Enable BGP LBW send for peering group DOWNSTREAM. 
+* Enable Link Bandwidth Cumulative feature on DOWNSTREAM.
+
+**TODO:** [Cumulative Link Bandwidth feature](https://datatracker.ietf.org/doc/draft-ietf-bess-ebgp-dmz/) is not currently modeled in OC. Related PR: https://github.com/openconfig/public/pull/1131
+
+
+2. Advertise the same test prefix from ATE from all UPSTREAM peers with LBW community:
+  * 32 peers - 10Mbps
+  * 16 peers - 20Mbps
+  * 8 peers - 40Mbps
+  * 8 peers - 80Mbps
+
+3. Verify that DUT advertises the test route to DOWNSTREAM eBGP peer with cumulative bandwidth community of 1600Mbps.
+
+### RT-7.6.2: Verify LBW changes.
+Using RT-7.6.1 set up conduct following changes:
+
+1) Disable 32 peers advertising 10Mpbs bandwidth community.
+2) Verify that DUT advertises the test route to Upstream peer with cumulative bandwidth community of 1280Mbps.
+3) Re-enable 32 peers advertising 10Mpbs bandwidth community.
+4) Verify that DUT advertises the test route to Upstream peer with cumulative bandwidth community of 1600Mbps.
+
+### RT-7.6.3: Verify LBW cumulative to iBGP peer
+
+1. Reconfigure 64x peers in peering group UPSTREAM to iBGP
+2. Reconfigure a single peer in peering group DOWNSTREAM to iBGP
+3. Repeat test RT-7.6.1 for iBGP.
+
+## OpenConfig Path and RPC Coverage
+
+```yaml
+paths:
+    /network-instances/network-instance/protocols/protocol/bgp/peer-groups/peer-group/afi-safis/afi-safi/use-multiple-paths/ebgp/link-bandwidth-ext-community/config/enabled:
+    /network-instances/network-instance/protocols/protocol/bgp/peer-groups/peer-group/afi-safis/afi-safi/use-multiple-paths/ibgp/link-bandwidth-ext-community/config/enabled:
+    # TODO: Add Cumulative LBW path.
+
+rpcs:
+  gnmi:
+    gNMI.Set:
+      union_replace: true
+      replace: true
+    gNMI.Subscribe:
+      on_change: true
+```
+
+## Required DUT platform
+
+* FFF