docs: warn that a relative local template path breaks copier update

Copier records the source argument verbatim as _src_path and resolves it
from the project directory at update time, so a render from a relative
local path is dead on the first copier update (copier-org/copier#335;
fix pending upstream in copier-org/copier#2717). Add a step-01 aside:
render from a URL or an absolute path, and hand-edit _src_path to
repair an existing render.

Issue: #373
Signed-off-by: Eduard R. <eduard@ralphovi.net>

Author: eduralph

docs/01-render-and-integrate.md

@@ -20,6 +20,16 @@ copier copy gh:eduralph/pdca-harness ../gramps-testbed-v2
 cd ../gramps-testbed-v2
 ```
 
+> **Rendering from a local checkout instead?** Give copier a `gh:`/`https:`
+> URL (as above) or an **absolute** path. Copier records the source argument
+> verbatim as `_src_path` in `.copier-answers.yml`, and `copier update`
+> resolves it from the *project* directory — a path relative to your render
+> cwd never resolves there, so the first `copier update` fails
+> ([copier#335](https://github.com/copier-org/copier/issues/335); fix pending
+> in [copier#2717](https://github.com/copier-org/copier/pull/2717)). Already
+> rendered that way? Hand-edit `_src_path` to the template URL or an absolute
+> path — that's the whole repair.
+
 The prompts (from [`copier.yml`](../copier.yml)) and what gramps answered:
 
 | Prompt | What it sets | gramps-testbed-v2 |