Updates to README files under the ext/ hierarchy. No changes to code.

ext/README.md

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+## Loadable Extensions
+
+Various [loadable extensions](https://www.sqlite.org/loadext.html) for
+SQLite are found in subfolders.
+
+Most subfolders are dedicated to a single loadable extension (for
+example FTS5, or RTREE).  But the misc/ subfolder contains a collection
+of smaller single-file extensions.

ext/misc/README.md

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+## Miscellaneous Extensions
+
+This folder contains a collection of smaller loadable extensions.
+See <https://www.sqlite.org/loadext.html> for instructions on how
+to compile and use loadable extensions.
+Each extension in this folder is implemented in a single file of C code.
+
+Each source file contains a description in its header comment.  See the
+header comments for details about each extension.  Additional notes are
+as follows:
+
+  *  **carray.c** &mdash;  This module implements the
+     [carray](https://www.sqlite.org/carray.html) table-valued function.
+     It is a good example of how to go about implementing a custom
+     [table-valued function](https://www.sqlite.org/vtab.html#tabfunc2).
+
+  *  **dbdump.c** &mdash;  This is not actually a loadable extension, but
+     rather a library that implements an approximate equivalent to the
+     ".dump" command of the
+     [command-line shell](https://www.sqlite.org/cli.html).
+
+  *  **memvfs.c** &mdash;  This file implements a custom
+     [VFS](https://www.sqlite.org/vfs.html) that stores an entire database
+     file in a single block of RAM.  It serves as a good example of how
+     to implement a simple custom VFS.
+
+  *  **rot13.c** &mdash;  This file implements the very simple rot13()
+     substitution function.  This file makes a good template for implementing
+     new custom SQL functions for SQLite.
+
+  *  **series.c** &mdash;  This is an implementation of the
+     "generate_series" [virtual table](https://www.sqlite.org/vtab.html).
+     It can make a good template for new custom virtual table implementations.
+
+  *  **shathree.c** &mdash;  An implementation of the sha3() and
+     sha3_query() SQL functions.  The file is named "shathree.c" instead
+     of "sha3.c" because the default entry point names in SQLite are based
+     on the source filename with digits removed, so if we used the name
+     "sha3.c" then the entry point would conflict with the prior "sha1.c"
+     extension.