Added : README within /lib directory, to help file selection

lib/README.md

@@ -0,0 +1,56 @@
+zstd - library files
+================================
+
+The __lib__ directory contains several files, but depending on target use case, some of them may not be necessary.
+
+#### Minimal library files
+
+##### Shared ressources
+
+- [mem.h](mem.h)
+- [error_private.h](error_private.h)
+- [error_public.h](error_public.h)
+
+##### zstd core compression
+
+- [bitstream.h](bitstream.h)
+- fse.c
+- fse.h
+- fse_static.h
+- huff0.c
+- huff0.h
+- huff0_static.h
+- zstd_compress.c
+- zstd_decompress.c
+- zstd_internal.h
+- zstd_opt.h
+- zstd.h
+- zstd_static.h
+
+#### Buffered streaming
+
+This complementary API makes streaming integration easier.
+It is used by `zstd` command line utility :
+
+- zbuff.c
+- zbuff.h
+- zbuff_static.h
+
+#### Dictionary builder
+
+To create dictionaries from training sets :
+
+- divsufsort.c
+- divsufsort.h
+- zdict.c
+- zdict.h
+- zdict_static.h
+
+#### Miscellaneous
+
+The other files are not source code. There are :
+
+ - LICENSE : contains the BSD license text
+ - Makefile : script to compile or install zstd library (static or dynamic)
+ - libzstd.pc.in : for pkg-config (make install)
+