add documentation about chunked I/O driver

doc/README.Chunk.md

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+# Support variable chunking and compression
+
+PnetCDF contains an experimental variable chunking and compression feature 
+for classic NetCDF files.
+
+For details about its design and implementation, please refer to:
+Hou, Kaiyuan, et al. "Supporting Data Compression in PnetCDF." 
+2021 IEEE International Conference on Big Data (Big Data). IEEE, 2021.
+
+## Enable variable chunking support
+
+* To build PnetCDF with variable chunking support
+  + Add `--enable-chunking` option at the configure command line. For example,
+    ```
+    ./configure --prefix=/PnetCDF/install/path --enable-chunking
+    ```
+* To build deflate filter support for chunked variable
+  + Add `--enable-zlib` option at the configure command line. Option
+    `--with-zlib` can also be used to specify the installation path of
+    zlib if it is not in the standard locations. For example,
+    ```
+    ./configure --prefix=/PnetCDF/install/path --enable-chunking --enable-zlib \
+                --with-zlib=/zlib/install/path
+    ```
+* To build sz filter support for chunked variable
+  + Add `--enable-sz` option at the configure command line. Option
+    `--with-sz` can also be used to specify the installation path of
+    sz if it is not in the standard locations. For example,
+    ```
+    ./configure --prefix=/PnetCDF/install/path --enable-chunking --enable-sz \
+                --with-sz=/sz/install/path
+    ```
+
+## Enable variable chunking
+
+To enable chunked storage layout for variables, set the file info "nc_chunking" 
+to "enable". The chunking feature requires 64-bit NetCDF format (CDF5). 
+For example,
+```
+    MPI_Info_create(&info);
+    ncmpi_create(MPI_COMM_WORLD, fname, NC_64BIT_DATA, info, &ncid);
+```
+Alternatively, the file info can be set through the environment variable 
+"PNETCDF_HINTS".
+```
+export PNETCDF_HINTS="nc_chunking=enable"
+```
+When chunking is enabled, all non-scalar variables will be stored in a chunked 
+storage layout. Scalar variables are not chunked.
+
+Users can also set the default filter for chunked variables. For example,
+```
+    MPI_Info_set(info, "nc_chunk_default_filter", "zlib");
+```
+or
+```
+export PNETCDF_HINTS="nc_chunking=enable;nc_chunk_default_filter=zlib"
+```
+The available filter options are none (default), zlib (deflate), sz.
+
+## Define chunk dimension of variables
+
+Applications can use the following APIs to set and get the chunk dimension of 
+a variable.
+```
+    int ncmpi_var_set_chunk (int ncid, int varid, int *chunk_dim);
+    int ncmpi_var_get_chunk (int ncid, int varid, int *chunk_dim);
+```
+For example:
+```
+    int dim[2] = {100, 100};
+    int chunk_dim[2] = {10, 10};
+    ncmpi_def_var (ncid, name, type, 2, dim, &varid)
+    ncmpi_var_set_chunk (ncid, varid, chunk_dim);
+```
+For record variables, the chunk dimension along the record dimension is always 
+1.
+The default chunk dimension is the dimension of the variable except for the 
+record dimension. By default, PnetCDF will create one chunk per record or 
+variable.
+
+## Define filter for chunked variables
+
+Applications can use the following APIs to set and get the chunk dimension of 
+a variable.
+```
+#define NC_FILTER_NONE  0
+#define NC_FILTER_DEFLATE   2
+#define NC_FILTER_SZ    3
+int ncmpi_var_set_filter (int ncid, int varid, int filter);
+int ncmpi_var_get_filter (int ncid, int varid, int *filter);
+```
+For example:
+```
+    ncmpi_var_set_filter (ncid, varid, NC_FILTER_DEFLATE);
+```
+Valid filter values are NC_FILTER_NONE (none), NC_FILTER_DEFLATE (zlib), and 
+NC_FILTER_SZ (sz).
+
+
+## Known problems
+
+There are some limitations of the experimental variable chunking feature.
+
+* Only one filter can be applied to a chunked variable. Unlike HDF5 which allows 
+  the stacking of multiple filters on chunked datasets, the current 
+  implementation in PnetCDF only allows a single filter to be applied to a 
+  variable.
+* No per-variable option for variable chunking. If chunking is enabled, all 
+  non-scalar variables will be chunked even if the chunk dimension is not 
+  defined.
+* Independent variable I/O is not supported. Variable read/write (get/put) 
+  must be collective in order to maintain data consistency of filtered chunks. 
+  Non-blocking APIs can be used to mitigate the impact of this limitation.
+
+Copyright (C) 2022, Northwestern University and Argonne National Laboratory
+
+See the COPYRIGHT notice in the top-level directory.
+