Add solver READMEs to documentation

[odow]

For JuMP 1.0, I think we should add the solver READMEs to the documentation. Something like this (I mocked it up for MOI, but it should be for JuMP):

This requires a 1.0 tag for each solver first so the API is stable. We don't want to release JuMP with the documentation baked in, and then releases a breaking version of the solver so that the documentation is out-of-date.

In doing so, we should standardize the README for each solver. At the moment solver documentation is lacking, and things like setting options aren't uniformly well documented. If the solver has proper documentation that's okay, the README can just link to that.

[joaquimg]

Not sure about standardizing README's. Solvers might be created totally independent of JuMP and just having a JuMP interface...
Users can just click to reach that info which is verbose anyway.
However, I think it would be interesting to add info to jump related to support of some standard constraints like: complements, indicator, SOS, Semiinteger/continuous, "main" cones (SDP, SOCP, Exp, Power). Of course, there could be a field for "more data".
Something in-between would be a standardized README like JuMP_README.md or even in a folder.

[odow]

We could standardize the README of solvers in jump-dev at least. Others are fine doing anything.

How about something like this:

# SolverName.jl

Short description.

[Badges]

## Installation

Install SolverName as follows:
```Julia
import Pkg
Pkg.add("SolverName")
```
Discuss any binary executables that need to be installed.

## The Optimizer object

Use `SolverName.Optimizer` to use SolverName with JuMP
or Convex.jl.

```Julia
using JuMP, SolverName
model = Model(SolverName.Optimizer)
```

Initialize SolverName with an environment as follows:
```Julia
using JuMP, SolverName
const env = SolverName.Env()
model = Model(() -> SolverName.Optimizer(env))
# or
model = Model() do
    return SolverName.Optimizer(env)
end
```

## Options

Common options for SolverName include

|------|---------|-------------|
| Name | Default | Description |
| logLevel | 0 | Set to 1 to print information to the screen |

## Supported constraint types

SolverName supports the following constraint types:

| ----- | ----- | 
| Function | Set |

## Low-level interface

SolverName provides complete access to the underlying C API.

All functions are identical to the C API and are exported. See the 
[documentation] for details.

Here is a basic example:
```Julia
ptr = solver_createProb()
ret = solver_solveModel(ptr, A, b, c)
print("Solved model. Status = $(ret)")
solver_freeProb(model)
```

[blegat]

We could standardize the README of solvers

+1000 for this.

About the "Supported constraint types" where would the solver say whether it support quadratic objective ?

[odow]

I've transferred this from JuMP to MathOptInterface. As part of the "implementing a solver interface" we should recommend a standard README.