Fix Pkg command for install pacakge from git

[nmoran]

In original README.md, the command used to install from git is Pkg.add, but should be Pkg.clone.

[fercarozzi]

Thanks for the contribution, but the tutorial is intended for general users, not developers. That is, users that would access the application as is. In addition, if the user is cloning the application for development, it should be Pkg.dev.

[nmoran]

Thanks for reviewing my PR, but I was talking about this from a users perspective and not a developers perspective. The instructions in the README as they are now don't work on OSX (with either julia 0.7 or 1.0.3). Entering Pkg.add("https://github.com/SeismicJulia/SeisMain.jl.git") gives

ERROR: https://github.com/SeismicJulia/SeisMain.jl.git is not a valid packagename
Stacktrace:
 [1] pkgerror(::String) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Pkg/src/Types.jl:120
 [2] check_package_name(::String) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Pkg/src/API.jl:22
 [3] iterate at ./generator.jl:47 [inlined]
 [4] collect(::Base.Generator{Array{String,1},typeof(Pkg.API.check_package_name)}) at ./array.jl:619
 [5] #add_or_develop#11 at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Pkg/src/API.jl:28 [inlined]
 [6] #add_or_develop at ./none:0 [inlined]
 [7] #add_or_develop#10 at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Pkg/src/API.jl:27 [inlined]
 [8] #add_or_develop at ./none:0 [inlined]
 [9] #add#18 at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Pkg/src/API.jl:69 [inlined]
 [10] add(::String) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/Pkg/src/API.jl:69
 [11] top-level scope at none:0

Using clone works and allows the package to be imported. From the documentation, seems a better way would be to add as a PackageSpec as

Pkg.add(PackageSpec(url="https://github.com/SeismicJulia/SeisMain.jl.git"))

[fercarozzi]

Pkg.add(PackageSpec(url="https://github.com/SeismicJulia/SeisMain.jl.git"))
is the correct way to go. "Pkg.add" alone would not work, you are right. I invite you to change your PR in this sense if you are interested to do so.
Again, thanks for your contribution.

[nmoran]

Thanks, made a followup commit which changes this to use PackageSpec.