docs: add readme for the decompose function

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Author: gunjjoshi

lib/node_modules/@stdlib/fft/base/fftpack/decompose/README.md

@@ -0,0 +1,145 @@
+<!--
+
+@license Apache-2.0
+
+Copyright (c) 2025 The Stdlib Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+-->
+
+# decompose
+
+> Factorize a sequence length into a product of integers.
+
+<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<!-- Package usage documentation. -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var decompose = require( '@stdlib/fft/base/fftpack/decompose' );
+```
+
+#### decompose( N, initial, out, stride, offset )
+
+Factorizes a sequence length into a product of integers.
+
+```javascript
+var initial = [ 3, 4, 2, 5 ]; // as found in FFTPACK
+var N = 630;
+var factors = [ 0, 0, 0, 0, 0, 0, 0 ];
+
+var numFactors = decompose( N, initial, factors, 1, 0 );
+// returns 5
+
+console.log( factors );
+// => [ 630, 5, 2, 3, 3, 5, 7 ]
+```
+
+The function accepts the following arguments:
+
+-   **N**: length of the sequence.
+-   **initial**: array of initial trial divisors.
+-   **out**: output array for storing factorization results.
+-   **stride**: stride length for `out`.
+-   **offset**: starting index for `out`.
+
+The function returns the number of factors into which `N` was decomposed.
+
+</section>
+
+<!-- /.usage -->
+
+<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="notes">
+
+## Notes
+
+-   Factorization results are stored in the output array as follows:
+
+    ```text
+    [ sequence_length | number_of_factors | integer_factors | unused_storage ]
+    ```
+
+-   The function mutates the input array.
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var decompose = require( '@stdlib/fft/base/fftpack/decompose' );
+
+var factors = [ 0, 0, 0, 0 ];
+var initial = [ 3, 4, 2, 5 ];
+var nf;
+var j;
+
+for ( j = 0; j < factors.length; j++ ) {
+    factors[ j ] = 0;
+}
+
+nf = decompose( 12, initial, factors, 1, 0 );
+
+console.log( 'Sequence length: %d', 12 );
+console.log( 'Number of factors: %d', nf );
+console.log( 'Factors:' );
+for ( j = 0; j < nf; j++ ) {
+    console.log( '  %d', factors[ j+2 ] );
+}
+```
+
+</section>
+
+<!-- /.examples -->
+
+<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="references">
+
+</section>
+
+<!-- /.references -->
+
+<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
+
+<section class="related">
+
+</section>
+
+<!-- /.related -->
+
+<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="links">
+
+</section>
+
+<!-- /.links -->