doc: added gather doc

examples/gather/README.md

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+# Gather
+
+This file provides an example of collective communication using gather across single and multiple worlds. This exaplme will perform gather 100 times on each rank from each world using a destination rank from a range from 0 to 2.
+
+`--worldinfo` argument is composed by the world index(1, 2) and the rank in that world (0, 1 or 2).
+
+## Running the Script in a Single World
+
+The single world example can be executed by opening 3 separate terminal windows to have 3 different processes and running the following commands in each terminal window:
+
+```bash
+# on terminal window 1 - will initialize 2 worlds (world1 and world2) with rank 0
+python m8d.py --backend nccl --worldinfo 1,0 --worldinfo 2,0
+# on terminal window 2 - will initialize world1 with rank 1
+python m8d.py --backend nccl --worldinfo 1,1
+# on terminal window 3 - will initialize world1 with rank 2
+python m8d.py --backend nccl --worldinfo 1,2
+```
+
+## Running the Script in Multiple Worlds
+
+The multiple world examplecan be executed by opening 5 separate terminal windows to have 5 different processes and running the following commands in each terminal window:
+
+```bash
+# on terminal window 1 - will initialize 2 worlds (world1 and world2) with rank 0
+python m8d.py --backend nccl --worldinfo 1,0 --worldinfo 2,0
+# on terminal window 2 - will initialize world1 with rank 1
+python m8d.py --backend nccl --worldinfo 1,1
+# on terminal window 3 - will initialize world1 with rank 2
+python m8d.py --backend nccl --worldinfo 1,2
+# on terminal window 4 - will initialize world2 with rank 1
+python m8d.py --backend nccl --worldinfo 2,1
+# on terminal window 5 - will initialize world2 with rank 2
+python m8d.py --backend nccl --worldinfo 2,2
+```
+
+To run processes on different hosts, `--addr` arugment can be used witn host's IP address. (`python m8d.py --backend nccl --worldinfo 1,0 --worldinfo 2,0 --addr 10.20.1.50`)