minor: only include master branch's MSRV in README (#321)

Author: patrickfreed

README.md

@@ -4,7 +4,8 @@
 This repository contains the officially supported MongoDB Rust driver, a client side library that can be used to interact with MongoDB deployments in Rust applications. It uses the [`bson`](https://docs.rs/bson) crate for BSON support. The driver contains a fully async API that supports either [`tokio`](https://crates.io/crates/tokio) (default) or [`async-std`](https://crates.io/crates/async-std), depending on the feature flags set. The driver also has a sync API that may be enabled via feature flag. 
 
 ## Index
-- [Installation](#Installation)
+- [Installation](#installation)
+    - [Requirements](#requirements)
     - [Importing](#importing)
         - [Configuring the async runtime](#configuring-the-async-runtime)
         - [Enabling the sync API](#enabling-the-sync-api)
@@ -26,16 +27,7 @@ This repository contains the officially supported MongoDB Rust driver, a client
 
 ## Installation
 ### Requirements
-| Driver Version | Required Rust Version |
-|:--------------:|:---------------------:|
-| master         | 1.47+                 |
-| 2.0.0-alpha    | 1.45+                 |
-| 1.2.x          | 1.43+                 |
-| 1.1.x          | 1.43+                 |
-| 1.0.x          | 1.43+                 |
-| 0.11.x         | 1.43+                 |
-| 0.10.x         | 1.43+                 |
-| 0.9.x          | 1.39+                 |
+- Rust 1.47+
 - MongoDB 3.6+
 
 **Note**: A bug affecting Rust 1.46-1.47 may cause out-of-memory errors when compiling an application that uses the 1.1
@@ -46,7 +38,7 @@ issue. For more information, see https://github.com/rust-lang/rust/issues/75992.
 The driver is available on [crates.io](https://crates.io/crates/mongodb). To use the driver in your application, simply add it to your project's `Cargo.toml`.
 ```toml
 [dependencies]
-mongodb = "1.1.1"
+mongodb = "2.0.0-alpha"
 ```
 
 #### Configuring the async runtime
@@ -55,7 +47,7 @@ The driver supports both of the most popular async runtime crates, namely [`toki
 For example, to instruct the driver to work with [`async-std`](https://crates.io/crates/async-std), add the following to your `Cargo.toml`:
 ```toml
 [dependencies.mongodb]
-version = "1.1.1"
+version = "2.0.0-alpha"
 default-features = false
 features = ["async-std-runtime"]
 ```
@@ -64,7 +56,7 @@ features = ["async-std-runtime"]
 The driver also provides a blocking sync API. To enable this, add the `"sync"` feature to your `Cargo.toml`:
 ```toml
 [dependencies.mongodb]
-version = "1.1.1"
+version = "2.0.0-alpha"
 default-features = false
 features = ["sync"]
 ```