readme: add source graph badge (#4980)

## Description

<img width="1140" alt="Screen Shot 2020-06-08 at 9 29 15 AM" src="https://user-images.githubusercontent.com/24299864/84003556-87e85800-a96a-11ea-828b-3656f116b518.png">


Closes: #XXX

README.md

@@ -12,6 +12,7 @@ Or [Blockchain](<https://en.wikipedia.org/wiki/Blockchain_(database)>), for shor
 [![Discord chat](https://img.shields.io/discord/669268347736686612.svg)](https://discord.gg/AzefAFd)
 [![license](https://img.shields.io/github/license/tendermint/tendermint.svg)](https://github.com/tendermint/tendermint/blob/master/LICENSE)
 [![](https://tokei.rs/b1/github/tendermint/tendermint?category=lines)](https://github.com/tendermint/tendermint)
+[![Sourcegraph](https://sourcegraph.com/github.com/tendermint/tendermint/-/badge.svg)](https://sourcegraph.com/github.com/tendermint/tendermint?badge)
 
 | Branch | Tests                                                                                                                                                                                                                                                  | Coverage                                                                                                                             | Linting                                                                    |
 | ------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------------------------------------- |