Create lgtm.yml for LGTM.com C/C++ analysis (facebook#4058)

Summary:
As discussed with thatsafunnyname [here](https://discuss.lgtm.com/t/c-c-lang-missing-for-facebook-rocksdb/1079): this configuration enables C/C++ analysis for RocksDB on LGTM.com.

The initial commit will contain a build command (simple `make`) that previously resulted in a build error. The build log will then be available on LGTM.com for you to investigate (if you like). I'll immediately add a second commit to this PR to correct the build command to `make static_lib`, which worked when I tested it earlier today.

If you like you can also enable automatic code review in pull requests. This will alert you to any new code issues before they actually get merged into `master`. Here's an example of how that works for the AMPHTML project: ampproject/amphtml#13060. You can enable it yourself here: https://lgtm.com/projects/g/facebook/rocksdb/ci/.

I'll also add a badge to your README.md in a separate commit — feel free to remove that from this PR if you don't like it.

(Full disclosure: I'm part of the LGTM.com team 🙂. Ping samlanning)
Closes facebook#4058

Differential Revision: D8648410

Pulled By: ajkr

fbshipit-source-id: 98d55fc19cff1b07268ac8425b63e764806065aa

Author: sj

.lgtm.yml

@@ -0,0 +1,4 @@
+extraction:
+  cpp:
+    index:
+      build_command: make static_lib

README.md

@@ -4,7 +4,6 @@
 [![Windows Build status](https://ci.appveyor.com/api/projects/status/fbgfu0so3afcno78/branch/master?svg=true)](https://ci.appveyor.com/project/Facebook/rocksdb/branch/master)
 [![PPC64le Build Status](http://140.211.168.68:8080/buildStatus/icon?job=Rocksdb)](http://140.211.168.68:8080/job/Rocksdb)
 
-
 RocksDB is developed and maintained by Facebook Database Engineering Team.
 It is built on earlier work on LevelDB by Sanjay Ghemawat ([email protected])
 and Jeff Dean ([email protected])