doc: add information about running the frontend with -local

Change-Id: I35a0b6194924a4e4d654af64e7bf2ef6be8bd902
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/263100
Reviewed-by: Julie Qiu <julie@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/doc/frontend.md b/doc/frontend.md
index 6b27903..2ed38e4 100644
--- a/doc/frontend.md
+++ b/doc/frontend.md
@@ -27,14 +27,15 @@
 
 You can run the frontend locally like so:
 
-    go run ./cmd/frontend [-dev] [-direct_proxy]
+    go run ./cmd/frontend [-dev] [-direct_proxy] [-local .]
 
 - The `-dev` flag reloads templates on each page load.
 
-The frontend can use one of two datasources:
+The frontend can use one of three datasources:
 
 - Postgres database
-- proxy service
+- Proxy service
+- Local filesystem
 
 The `Datasource` interface implementation is available at internal/datasource.go.
 
@@ -49,6 +50,12 @@
 
 You can then run the frontend with: `go run ./cmd/frontend`
 
+You can also use `-local` flag to run the frontend with an in-memory datasource
+populated with modules loaded from your local filesystem. This allows you to run
+the frontend without setting up a database and to view documentation of local
+modules without requiring a proxy. `-local` accepts a GOPATH-like string containing
+paths of modules to load into memory.
+
 If you add, change or remove any inline scripts in templates, run
 `devtools/cmd/csphash` to update the hashes. Running `all.bash`
 will do that as well.