Add document for "Double Buffer" example

examples/blockStreaming_doubleBuffer.md

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+# LZ4 Streaming API Example : Double Buffer
+
+`blockStreaming_doubleBuffer.c` is LZ4 Straming API example which implements double buffer (de)compression.
+
+Please note :
+
+ - Firstly, read "LZ4 Streaming API Basics".
+ - This is relatively advanced application example.
+ - Output file is not compatible with lz4frame and platform dependent.
+
+
+## What's the point of this example ?
+
+ - Handle huge file in small amount of memory
+ - Always better compression ratio than Block API
+ - Uniform block size
+
+
+## How the compression works
+
+First of all, allocate "Double Buffer" for input and LZ4 compressed data buffer for output.
+Double buffer has two pages, "first" page (Page#1) and "second" page (Page#2).
+
+```
+        Double Buffer
+
+      Page#1    Page#2
+    +---------+---------+
+    | Block#1 |         |
+    +----+----+---------+
+         |
+         v
+      {Out#1}
+
+
+      Prefix Dependency
+         +---------+
+         |         |
+         v         |
+    +---------+----+----+
+    | Block#1 | Block#2 |
+    +---------+----+----+
+                   |
+                   v
+                {Out#2}
+
+
+   External Dictionary Mode
+         +---------+
+         |         |
+         |         v
+    +----+----+---------+
+    | Block#3 | Block#2 |
+    +----+----+---------+
+         |
+         v
+      {Out#3}
+
+
+      Prefix Dependency
+         +---------+
+         |         |
+         v         |
+    +---------+----+----+
+    | Block#3 | Block#4 |
+    +---------+----+----+
+                   |
+                   v
+                {Out#4}
+```
+
+Next, read first block to double buffer's first page. And compress it by `LZ4_compress_continue()`.
+For the first time, LZ4 doesn't know any previous dependencies,
+so it just compress the line without dependencies and generates compressed block {Out#1} to LZ4 compressed data buffer.
+After that, write {Out#1} to the file.
+
+Next, read second block to double buffer's second page. And compress it.
+In this time, LZ4 can use dependency to Block#1 to improve compression ratio.
+This dependency is called "Prefix mode".
+
+Next, read third block to double buffer's *first* page. And compress it.
+Also this time, LZ4 can use dependency to Block#2.
+This dependency is called "External Dictonaly mode".
+
+Continue these procedure to the end of the file.
+
+
+## How the decompression works
+
+Decompression will do reverse order.
+
+ - Read first compressed block.
+ - Decompress it to the first page and write that page to the file.
+ - Read second compressed block.
+ - Decompress it to the second page and write that page to the file.
+ - Read third compressed block.
+ - Decompress it to the *first* page and write that page to the file.
+
+Continue these procedure to the end of the compressed file.