Add git-subtree documentation.

README.adoc

@@ -1,10 +1,12 @@
 = curl_wrapper
+:toc: preamble
 :project: curl_wrapper
 :uri-base: https://schlomp.space/tastytea/{project}
 :uri-cmake: https://cmake.org/
 :uri-libcurl: https://curl.haxx.se/libcurl/
 :uri-catch: https://github.com/catchorg/Catch2
 :uri-doxygen: http://www.doxygen.nl/
+:uri-git-subtree: https://manpages.debian.org/testing/git-man/git-subtree.1.en.html
 
 Light libcurl wrapper for when you need to GET a website with minimum effort.
 
@@ -55,6 +57,24 @@ target_link_libraries(test PRIVATE curl_wrapper)
 * `-DWITH_CURL_WRAPPER_TESTS=YES` Compiles the tests.
 * `-DWITH_CURL_WRAPPER_DOC=YES` Generate API reference.
 
+== Add with `git subtree`
+
+Replace `src/curl_wrapper` with the path you want it to be in. Consult
+link:{uri-git-subtree}[git-subtree(1)] for more information.
+
+[source,shell]
+--------------------------------------------------------------------------------
+git remote add curl_wrapper https://schlomp.space/tastytea/curl_wrapper.git
+git subtree add --prefix=src/curl_wrapper curl_wrapper main --squash
+--------------------------------------------------------------------------------
+
+=== Update subtree
+
+[source,shell]
+--------------------------------------------------------------------------------
+git subtree pull --prefix=src/curl_wrapper curl_wrapper main --squash
+--------------------------------------------------------------------------------
+
 == Dependencies
 
 * C++17