mojo_run: infer --origin from `MOJO_VERSION` file if present.

mojo/devtools/common/docs/mojo_run.md

Index: mojo/devtools/common/docs/mojo_run.md
diff --git a/mojo/devtools/common/docs/mojo_run.md b/mojo/devtools/common/docs/mojo_run.md
index 11758b68023231a2e84300f17659758530d302a8..ad4f0e269a81ddb924bb55ff239181b6e20d7e89 100644
--- a/mojo/devtools/common/docs/mojo_run.md
+++ b/mojo/devtools/common/docs/mojo_run.md
@@ -69,3 +69,12 @@ and the following in another:
 mkdir ~/another_home
 HOME=~/another_home mojo_run APP_URL --reuse-servers
 ```
+
+## Setting default mojo origin
+
+When run outside of the `domokit/mojo` repository, `mojo_run` needs `--origin`
+parameter to indicate where binaries of the core mojo services come from. If a
+`MOJO_VERSION` file is present among ancestors of `mojo_run` and `--origin`
+parameter is not set, origin will point to Google Storage location storing
+binaries of core mojo services built at the git revision indicated in
+`MOJO_VERSION`.