Add cirrus ci badge to README for freebsd build

Also only build stable toolchain only - cirrus limits us to 2 concurrent freebsd
tasks at a time, and having 3 toolchains being built means that we were spending
extra time queued

.cirrus.yml

@@ -6,8 +6,6 @@ freebsd_task:
   env:
     matrix:
       - TOOLCHAIN: stable
-      - TOOLCHAIN: beta
-      - TOOLCHAIN: nightly
   setup_script:
     - pkg install -y curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh

README.md

@@ -1,7 +1,8 @@
-py-spy: A sampling profiler for Python programs.
+py-spy: Sampling profiler for Python programs
 =====
 [![Build Status](https://travis-ci.org/benfred/py-spy.svg?branch=master)](https://travis-ci.org/benfred/py-spy)
 [![Windows Build status](https://ci.appveyor.com/api/projects/status/x0jwd5ygaybsa0md?svg=true)](https://ci.appveyor.com/project/benfred/py-spy)
+[![FreeBSD Build Status](https://api.cirrus-ci.com/github/benfred/py-spy.svg)](https://cirrus-ci.com/github/benfred/py-spy)
 
 py-spy is a sampling profiler for Python programs. It lets you visualize what your Python
 program is spending time on without restarting the program or modifying the code in any way.