message: Add README for base-host
diff: # @fluidframework/base-host
  +
  +`BaseHost` provides an easy-to-use entry point for hosts of Fluid experiences. Given a configuration, it helps load and initialize a container and provides helpers to retrieve components from that container.
  +
  +## BaseHost
  +
  +### Creation
  +
  +To create a `BaseHost`, you'll need to profide a configuration `IBaseHostConfig`.
  +
  +```typescript
  +const baseHostConfig = {
  + codeResolver, // an IFluidCodeResolver
  + documentServiceFactory, // an IDocumentServiceFactory
  + urlResolver, // an IUrlResolver
  +};
  +const baseHost = new BaseHost(baseHostConfig);
  +```
  +
  +#### IBaseHostConfig
  +
  +The key members of `IBaseHostConfig` are an `IFluidCodeResolver` (used for loading code into the container), an `IDocumentServiceFactory` (used for connecting to the Fluid service), and an `IUrlResolver` (used for resolving URLs used in API calls against the `Loader` and `Container`).
  +
  +### Usage
  +
  +`BaseHost` provides a method `.initializeContainer()` which will retrieve a `Container` from the given url, and if necessary initializing it with the given code in the process.
  +
  +```typescript
  +const container = await baseHost.initializeContainer(url, codeDetails);
  +```
  +
  +Once the container is retrieved and initialized this way, requests can be made against it.
  +
  +`BaseHost` also provides a method `.getComponent()` for retrieving components directly (bypassing the `Container`) for convenience.
  +
  +```typescript
  +const component = await baseHost.getComponent(url);
  +```
  +
  +The `Loader` can be also be retrieved via `.getLoader()`.
  +
  +```typescript
  +const loader = await baseHost.getLoader();
  +```
  +
  +## initializeContainerCode()
  +
  +If the full facilities of a `BaseHost` aren't needed, the helper `initializeContainerCode()` can be used directly to initialize a container with code.
  +
  +```typescript
  +await initializeContainerCode(container, codeDetails);
  +```
  +
  +After this promise resolves, the container will be initialized, though the context change may not have occurred yet.
  