Add Technical Writing subject with one book (#11033)

* add technical writing subject

* Update free-programming-books-subjects.md

books/free-programming-books-subjects.md

@@ -654,6 +654,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [Scientific Programming and Computer Architecture](https://divakarvi.github.io/bk-spca/spca.html) - Divakar Viswanath
 * [Signal Computing: Digital Signals in the Software Domain](http://faculty.washington.edu/stiber/pubs/Signal-Computing/Signal%20Computing.pdf) - Michael Stiber, Bilin Zhang Stiber, Eric C. Larson (PDF)
 * [Small Memory Software](https://smallmemory.charlesweir.com/book.html) - Charles Weir, James Noble (HTML)
+* [Software Technical Writing: A Guidebook](https://jamesg.blog/book.pdf) - James Gallagher (PDF)
 * [Web Almanac](https://almanac.httparchive.org/static/pdfs/web_almanac_2019_en.pdf) (PDF)
 * [Writing Native Mobile Apps in a Functional Language Succinctly](https://www.syncfusion.com/ebooks/writing_native_mobile_apps_in_a_functional_language_succinctly) - Vassili Kaplan