(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

README.md

@@ -45,12 +45,20 @@ For building from source, you can start with this:
     npm install
     npm run build:prod
     npm run install:python
-    GRIST_DEFAULT_EMAIL=you@example.com npm start
+    npm start
     # unauthenticated grist api available at http://localhost:8484/api/
 
-Stay tuned for more instructions to come at release.
+Then you can use the Grist API locally to work on a document. Currently you need
+to "upload" a document to work with it. This makes a copy of it that the server
+controls - you can find it in the `data` directory:
 
-For using pre-built Grist, just head on over to <https://www.getgrist.com>.
+    curl -F 'upload=@YourDocument.grist' http://localhost:8484/api/docs
+    # Note the document ID that is returned.
+
+The [Data-Tables endpoints](https://support.getgrist.com/api/#tag/Data-Tables) are
+particularly useful.
+
+For using hosted Grist, just head on over to <https://www.getgrist.com>.
 
 # License
 

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));
 }