Add guide bool_plperl (#459)

Co-authored-by: Evan Stanton <[email protected]>

contrib/bool_plperl/Guide.md

@@ -0,0 +1,48 @@
+# What is this extension?
+>*Transform boolean type <> PL/Perl.*
+
+`bool_plperl` is a C-based transformation extension that allows conversion between PostgreSQL- and Perl-native boolean values.
+
+# When should you use it?
+
+`bool_plperl` should be considered when writing or using PL/Perl functions within PostgreSQL that involve boolean values. This extension ensures that boolean values from PostgreSQL are properly interpreted in Perl and vice versa, eliminating potential logical errors.
+
+# Example use case.
+
+Library:
+
+A public library wants to check if a particular book is available for borrowing by means of a PL/Perl function. Due to the boolean value interpretation differences between PostgreSQL and Perl, they use the `bool_plperl` extension to make this process seamless.
+
+# Example test script.
+
+```
+-- Sample table for library books
+CREATE TABLE library_books (
+    book_id serial PRIMARY KEY,
+    title text,
+    is_available bool
+);
+
+-- Populate the table with a sample book
+INSERT INTO library_books (title, is_available) VALUES 
+('The Great Gatsby', true),
+('To Kill a Mockingbird', false),
+('Brothers Grimm Fairy Tales', true);
+
+-- Create a PL/Perl function to check book availability
+CREATE FUNCTION is_book_available(book_title text) RETURNS bool
+TRANSFORM FOR TYPE bool
+AS $$
+  my ($book_title) = @_;
+  my $rv = spi_exec_query("SELECT is_available FROM library_books WHERE title = '$book_title'");
+  return $rv->{rows}[0]->{is_available};
+$$ LANGUAGE plperl;
+
+-- Test the function
+SELECT title, is_book_available(title) as available_status FROM library_books WHERE title = 'The Great Gatsby';
+
+title             | available_status
+------------------+------------------
+ The Great Gatsby | t
+(1 row)
+```