Add src/README.txt and build/README.txt

[mkoeppe]

This ticket adds README files that explain the purpose of the src and build directories.

Also, src/setup.py sdist (#21516) is asking for a README, so we make it happy too.

CC: @vbraun @jdemeyer @embray @kiwifb @dimpase @fchapoton

Component: build

Author: Matthias Koeppe

Branch/Commit: 1b45e56

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/21565

[mkoeppe]

Branch: u/mkoeppe/add_src_readme_txt_and_build_readme_txt

[mkoeppe]

Commit: 39f5eb9

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

comment:2

Needs review. New commits that improve/expand these descriptions are, of course, welcome too.

---

New commits:

39f5eb9

Add READMEs

[sagetrac-git]

Changed commit from 39f5eb9 to 1b45e56

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

1b45e56

Fix typo in README

[dimpase]

Reviewer: Dima Pasechnik

[dimpase]

comment:5

looks good enough to me (I'm a bit unhappy about .txt suffix vs no such suffix in other READMEs, but OK).

[mkoeppe]

comment:6

Replying to @dimpase:

looks good enough to me

Is this a "positive review"?

I'm a bit unhappy about .txt suffix vs no such suffix in other READMEs, but OK

Note the majority of our README files already has .txt as a suffix.

$ find build -name README\*
build/pkgs/atlas/patches/ATLAS-lib/README
build/pkgs/bzip2/patches/autotools/README.autotools
build/pkgs/giac/patches/README.txt
build/pkgs/pari/patches/README.txt
build/sage_bootstrap/README
$ find src -name README\*
src/mac-app/tools/createDSStore/README.txt
src/sage/ext/mac-app/README
src/sage/libs/gap/test/README.txt
src/sage/rings/bernmm/README.txt
src/sage/rings/bernmm/README2.txt
src/sage/rings/padics/README.txt
src/sage/schemes/hyperelliptic_curves/hypellfrob/README
src/sage/tests/french_book/README

[dimpase]

comment:7

OK - was in hurry and did not notice that I didn't set it to positive review, sorry.

[vbraun]

Changed branch from u/mkoeppe/add_src_readme_txt_and_build_readme_txt to 1b45e56