Add Project.toml (and ignore Manifest.toml)

Here is how I created it:

```
julia> Pkg.API.project("ForwardDiff", ".."; preview=false)
julia> edit("Project.toml")
(v0.7) pkg> activate .
(ForwardDiff) pkg> add StaticArrays DiffResults DiffRules NaNMath SpecialFunctions CommonSubexpressions
```

uuid is taken from:
https://github.com/JuliaRegistries/General/blob/5effb80270af0c849135c80c4fdbcfe9a1a5c180/F/ForwardDiff/Package.toml#L2

Author: tkf

.gitignore

@@ -5,3 +5,4 @@
 docs/build/
 docs/site/
 benchmark_data/
+Manifest.toml

Project.toml

@@ -0,0 +1,10 @@
+name = "ForwardDiff"
+uuid = "f6369f11-7733-5829-9624-2563aa707210"
+
+[deps]
+CommonSubexpressions = "bbf7d656-a473-5ed7-a52c-81e309532950"
+DiffResults = "163ba53b-c6d8-5494-b064-1a9d43ac40c5"
+DiffRules = "b552c78f-8df3-52c6-915a-8e097449b14b"
+NaNMath = "77ba4419-2d1f-58cd-9bb1-8ffee604a2e3"
+SpecialFunctions = "276daf66-3868-5448-9aa4-cd146d93841b"
+StaticArrays = "90137ffa-7385-5640-81b9-e52037218182"