(core) simplify starting grist-core and add some more usage information

Summary:
I worked through the README for grist-core, and the instructions
for setting it up and starting it.  This change includes a small
simplification, and a few more instructions for getting started.

Test Plan: manual

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2619

stubs/app/server/server.ts

@@ -1,5 +1,7 @@
 /**
  * Main entrypoint for grist-core server.
+ *
+ * By default, starts up on port 8484.
  */
 
 import {updateDb} from 'app/server/lib/dbUtils';
@@ -10,20 +12,28 @@ const G = {
   port: parseInt(process.env.PORT!, 10) || 8484,
 };
 
+// Set a default for an environment variable.
+function setDefaultEnv(name: string, value: string) {
+  if (process.env[name] === undefined) {
+    process.env[name] = value;
+  }
+}
+
 export async function main() {
   // Use a distinct cookie.
-  if (!process.env.GRIST_SESSION_COOKIE) {
-    process.env.GRIST_SESSION_COOKIE = 'grist_core';
-  }
-  // This is where documents are placed, for historic reasons.
-  await fse.mkdirp('samples');
+  setDefaultEnv('GRIST_SESSION_COOKIE', 'grist_core');
+  // There's no login system released yet, so set a default email address.
+  setDefaultEnv('GRIST_DEFAULT_EMAIL', 'support@getgrist.com');
+  // Set directory for uploaded documents.
+  setDefaultEnv('GRIST_DATA_DIR', 'data');
+  await fse.mkdirp(process.env.GRIST_DATA_DIR!);
   // Make a blank db if needed.
   await updateDb();
   // Launch single-port, self-contained version of Grist.
-  // You probably want to have GRIST_DEFAULT_EMAIL set since there's no login system yet.
   await mergedServerMain(G.port, ["home", "docs", "static"]);
 }
 
 if (require.main === module) {
+  // tslint:disable-next-line:no-console
   main().catch((err) => console.error(err));
 }