docs: how to get old versions of ckb

docs/get-ckb.md

@@ -5,6 +5,9 @@
 We will publish binaries for each release via [Github Releases](https://github.com/nervosnetwork/ckb/releases). If your system
 is listed there, you can download the package directory.
 
+There is also a repository [ckb-nightly](https://github.com/nervosnetwork/ckb-nightly/releases) containing the nightly builds from the develop
+branch.
+
 
 ## Build from Source
 
@@ -41,7 +44,11 @@ sudo pacman -Sy git gcc pkgconf clang
 brew install autoconf libtool
 ```
 
-### Build from master branch
+### Build from source
+
+The `master` branch is regularly built and tested. It is always the latest released version. The default checked out branch `develop` is the latest version in development.
+
+It is recommended to build a version from master.
 
 ```bash
 # get ckb source code
@@ -56,8 +63,15 @@ make build
 This will build the executable `target/release/ckb`. Please add the directory
 to `PATH` or copy/link the file into a directory already in the `PATH`.
 
-```base
+```bash
 export PATH="$(pwd)/target/release:$PATH"
 # or
 # ln -snf "$(pwd)/target/release/ckb" /usr/local/bin/ckb
 ```
+
+It is easy to switch to a history version and build, for example, check out
+v0.8.0:
+
+```bash
+git checkout -b branch-v0.8.0 v0.8.0
+```