Add Building info

Signed-off-by: Josh Cepek <josh.cepek@usa.net>

build/Building.md

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+Building Easy-RSA 3
+===
+
+Until a formal buildscript is available, this document serves as the build
+reference. Note that Windows builds require external binary files from the
+unxutils and mksh/Win32 projects which are not included in the Easy-RSA source
+tree.
+
+Basic Builds
+---
+
+This build sequence applies to all platforms, including Windows.
+
+1. Checkout the release tag of interest
+
+2. Create a target directory named EasyRSA-3.x.y (matching the release version)
+
+3. Copy the `easyrsa3/*` structure to the target dir root (not named easyrsa3)
+
+4. Copy the following files/dirs into the target dir:
+
+   * `Licensing/`
+   * `doc/`
+   * `COPYING`
+   * `ChangeLog`
+   * `README.quickstart.md`
+
+5. For Windows, continue to `Windows Build Extras`. Otherwise, tar up the target
+   directory for release distribution.
+
+Windows Build Extras
+---
+
+Windows has additional build steps to provide a suitable POSIX environment.
+Starting with a basic build dir from earlier, proceed as follows.
+
+1. Copy the following files/dirs into the target dir. Unless specified, all
+   source files or referenced dirs go into the root of the target dir. Take care
+   that txt/bat files follow Windows EOL convention (CR+LF.)
+
+   * `distro/windows/EasyRSA Start.bat`
+   * `distro/windows/doc/README-Windows.txt`
+   * `distro/windows/bin/`
+   * `distro/windows/Licensing/`
+
+2. Convert the .md readme/doc files into html for easier viewing by Windows
+   clients. One option using the python3 `markdown` module is:
+
+    find ./ -name '*.md' | while read f
+    do
+      python3 -m markdown "$f"  "${f/.md/.html}"
+      rm "$f"
+    done
+
+3. Copy mksh.exe from the mksh/Win32 project into the target dir named exactly
+   `bin/sh.exe` (note the name difference.)
+
+4. Copy the following files from the unxutils project into the target `/bin/`
+   dir. Files marked with [+] are optional in unofficial builds and serve only
+   to make the shell environment more usable for users.
+
+   * awk.exe
+   * cat.exe
+   * cp.exe
+   * diff.exe [+]
+   * grep.exe [+]
+   * ls.exe [+]
+   * md5sum.exe [+]
+   * mkdir.exe
+   * mv.exe [+]
+   * printf.exe
+   * rm.exe
+   * sed.exe [+]
+   * which.exe
+
+5. Zip up the target directory for release distribution.