(core) Add an initial README to grist-core.

Test Plan: No code changes to test

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2566

README.md

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+# Grist
+
+Grist is a modern relational spreadsheet. It combine the flexibility of a spreadsheet with the
+robustness of a database to organize your data and make you more productive.
+
+> :warning: This repository is in a pre-release state. Its release will be announced when it has
+all the planned components, and a solid independent build and test set-up. Currently, stand-alone
+server functionality is present. Release of our web client is planned, along with an extensive
+test suite.
+
+This repository, [grist-core](https://github.com/gristlabs/grist-core), is maintained by Grist
+Labs. Our flagship product, available at https://www.getgrist.com, is built from the code you see
+here, combined with business-specific software designed to scale it to many users, handle billing,
+etc.
+
+## Why Open Source?
+
+By opening its source code and offering an [OSI](https://opensource.org/)-approved free license,
+Grist benefits its users:
+
+- **Open Source Community.** An active community is the main draw of open-source projects. Anyone
+  can examine source code, and contribute bug fixes or even new features. This is a big deal for a
+  general-purpose spreadsheet-like product, where there is a long tail of features vital to
+  someone somewhere.
+- **Increased Trust.** Because anyone can examine the source code, “security by obscurity” is not
+  an option. Vulnerabilities in the code can be found by others and reported before they can cause
+  damage.
+- **Independence.** The published source code—and the product built from it—are available to you
+  regardless of the fortunes of the Grist Labs business. Whatever happens to us, this repo or its
+  forks can live on, so that you can continue to work on your data in Grist.
+- **Price Flexibility.** You can build Grist from source and use it for yourself all you want
+  without paying us a cent. While you can’t go wrong with our fully set-up and supported online
+  service, some organizations may choose the do-it-yourself route and pay for their own server and
+  maintenance, rather than a per-user price. DIY users are often the ones to develop new features,
+  and can contribute them back to benefit all users of Grist.
+- **Extensibility.** For developers, having the source open makes it easier to build extensions (such as the
+  experimental [Custom Widget](https://support.getgrist.com/widget-custom/)). You can more easily
+  include Grist in your pipeline. And if a feature is missing, you can just take the source code and
+  build on top of it!
+
+## How do I start?
+
+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
+    # unauthenticated grist api available at http://localhost:8484/api/
+
+Stay tuned for more instructions to come at release.
+
+For using pre-built Grist, just head on over to <https://www.getgrist.com>.