Add doc/std/README.md

doc/std/README.md

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+# Standard Library
+
+Wuffs' standard library consists of multiple packages, each implementing a
+particular file format or algorithm. Those packages can be grouped into several
+categories:
+
+- [Compression Decoders](/doc/std/compression-decoders.md).
+- [Hashers](/doc/std/hashers.md).
+- [Image Decoders](/doc/std/image-decoders.md).
+
+The general pattern is that a package `foo` (e.g. `jpeg`, `png`) contains a
+struct `bar` (e.g. `hasher`, `decoder`, etc) that implements a
+package-independent interface. For example, every compression decoder struct
+would have a `decode_io_writer` method. In C, this would be invoked as
+
+```
+// Allocate and initialize the struct.
+wuffs_foo__decoder* dec = etc;
+
+// Do the work. Error checking is not shown, for brevity.
+const char* status = wuffs_foo__decoder__decode_io_writer(dec, etc);
+```
+
+When that C library is used as C++, that last line can be shortened:
+
+```
+const char* status = dec->decode_io_writer(etc);
+```
+
+See also the [glossary](/doc/glossary.md), as well as the notes on:
+
+- [Coroutines](/doc/note/coroutines.md).
+- [Initialization](/doc/note/initialization.md).
+- [Statuses](/doc/note/statuses.md).

doc/std/hashers.md

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 ## Examples
 
 - [example/crc32](/example/crc32)
+
+
+---
+
+See also the general remarks on [Wuffs' standard library](/doc/std/README.md).