(doc): add compression documentation part of #402

Documentation/compression.md

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+# Compression
+
+## Client side
+The preferred method for configuring message compression on a client is to pass `options` when the client object is instantiated.
+
+The two options need to be passed:
+
+**grpc.default_compression_algorithm** (integer)
+
+The option keeps the value of a compression algorithm.
+
+Possible values for this option are:
+- `0` - No compression
+- `1` - Compress with DEFLATE algorithm
+- `2` - Compress with GZIP algorithm
+- `3` - Stream compression with GZIP algorithm
+
+**grpc.default_compression_level** (integer)
+
+The option keeps the value for the level of compression.
+
+Possible values for this option are:
+- `0` - None
+- `1` - Low level
+- `2` - Medium level
+- `3` - High level
+
+### Code example
+```javascript
+client = new ExampleClient("example.com", credentials.createInsecure(), {'grpc.default_compression_algorithm': 2, 'grpc.default_compression_level': 2});
+```