clarify README.md

[wds15]

Summary

Adapt README.md to our new logic of instructions.

Tests

Side Effects

Release notes

clarify README.md

Checklist

• Math issue Update README.md #1820

• Copyright holder: Sebastian Weber

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[wds15]

Hopefully Jenkins will not change back this formatting thing (which pops up in all PRs right now for whatever reason).

[rok-cesnovar]

Yes some Jenkins workers have clang-format 5.0.0 and some have 6.0.0 and they differ in some details and that needs to be sorted out. I will make an issue and assign Nic.

[rok-cesnovar]

THe changes look good to me.

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	4.92	4.92	1.0	-0.09% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.96	-4.21% slower
eight_schools/eight_schools.stan	0.09	0.09	1.0	0.19% faster
gp_regr/gp_regr.stan	0.22	0.22	0.99	-0.62% slower
irt_2pl/irt_2pl.stan	6.55	6.49	1.01	0.96% faster
performance.compilation	89.06	87.05	1.02	2.26% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	7.57	7.55	1.0	0.33% faster
pkpd/one_comp_mm_elim_abs.stan	20.51	20.68	0.99	-0.8% slower
sir/sir.stan	96.41	92.87	1.04	3.67% faster
gp_regr/gen_gp_data.stan	0.05	0.05	1.01	0.85% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.95	2.96	1.0	-0.26% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.32	0.31	1.02	1.49% faster
arK/arK.stan	1.91	1.9	1.0	0.38% faster
arma/arma.stan	0.65	0.66	0.98	-1.67% slower
garch/garch.stan	0.51	0.51	0.99	-0.51% slower
Mean result: 1.00160872994

---

Jenkins Console Log
Blue Ocean
Commit hash: f204168

---

Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix