Installation issues

There are some issues in the installation guidelines

- (easy to fix) There is a missing dependency with the dapp tool
- The 'MakeFile' expect one json file per solidity file. However 'dapp' produces only one json file ('dss/out/dapp.sol.json').

For instance after running 'make build' I get:

```
dapp --version
dapp 0.24.0
solc, the solidity compiler commandline interface
Version: 0.5.10+commit.5a6ea5b1.Darwin.clang
hevm 0.33
git submodule update --init --recursive
./k-dss/dss && dapp --use solc:0.5.9 build && cd ../
+ dapp clean
+ rm -rf out
+ solc --overwrite ds-test/=lib/ds-test/src/ ds-test=lib/ds-test/src/index.sol ds-token/=lib/ds-token/src/ ds-token=lib/ds-token/src/index.sol ds-value/=lib/ds-value/src/ ds-value=lib/ds-value/src/index.sol erc20/=lib/ds-token/lib/erc20/src/ erc20=lib/ds-token/lib/erc20/src/index.sol ds-math/=lib/ds-token/lib/ds-math/src/ ds-math=lib/ds-token/lib/ds-math/src/index.sol ds-stop/=lib/ds-token/lib/ds-stop/src/ ds-stop=lib/ds-token/lib/ds-stop/src/index.sol ds-thing/=lib/ds-value/lib/ds-thing/src/ ds-thing=lib/ds-value/lib/ds-thing/src/index.sol ds-auth/=lib/ds-token/lib/ds-stop/lib/ds-auth/src/ ds-auth=lib/ds-token/lib/ds-stop/lib/ds-auth/src/index.sol ds-note/=lib/ds-token/lib/ds-stop/lib/ds-note/src/ ds-note=lib/ds-token/lib/ds-stop/lib/ds-note/src/index.sol --combined-json=abi,bin,bin-runtime,srcmap,srcmap-runtime,ast,metadata /=/ src/dai.sol src/jug.sol src/test/flip.t.sol src/test/vat.t.sol src/test/vow.t.sol src/test/flap.t.sol src/test/flop.t.sol src/test/dai.t.sol src/test/pot.t.sol src/test/end.t.sol src/test/fork.t.sol src/test/jug.t.sol src/flap.sol src/spot.sol src/join.sol src/vat.sol src/end.sol src/lib.sol src/vow.sol src/pot.sol src/cat.sol src/flip.sol src/flop.sol

...
fs.js:125
throw err;
^

Error: ENOENT: no such file or directory, open './dss/out/vat.sol.json'
at Object.openSync (fs.js:454:3)
at Object.readFileSync (fs.js:354:35)
...
```

When trying to run the 'Makefile' in 'dss' folder it also fails.

```
make build
rm -rf out/
mkdir -p out/
solc --overwrite ds-note/=lib/ds-note/src/ ds-test/=lib/ds-test/src/ ds-token/=lib/ds-token/src/ ds-value/=lib/ds-value/src/ erc20/=lib/ds-token/lib/erc20/src/ ds-math/=lib/ds-token/lib/ds-math/src/ ds-stop/=lib/ds-token/lib/ds-stop/src/ ds-thing/=lib/ds-value/lib/ds-thing/src/ ds-auth/=lib/ds-token/lib/ds-stop/lib/ds-auth/src/ --combined-json=abi,bin,bin-runtime,srcmap,srcmap-runtime,ast /=/ $(find src -type f -name '*.sol') > out/dss.json
lib/ds-token/lib/ds-stop/src/stop.sol:21:1: Error: Source "lib/ds-note/src/note.sol" not found: File not found.
import "ds-note/note.sol";
^------------------------^
lib/ds-value/lib/ds-thing/src/thing.sol:21:1: Error: Source "lib/ds-note/src/note.sol" not found: File not found.
import 'ds-note/note.sol';
^------------------------^
make: *** [build] Error 1
```

Start with the installation guidelines and the Makefiles in the repository root and the dss folder, then reproduce the two `make build` failures shown in the issue. Trace the missing dapp dependency, the expected per-file JSON output, and the missing `ds-note` import. Done means the documented installation and both build commands complete successfully with the expected output.

Written by the indexing model from the issue text.