Move to .md files for even trivial documentation.

So it's automatically displayed for folks browsing the source.

Bug: N/A
Test: N/A
Change-Id: I80e823415f1fb12ad8ce5348e926519b6e29580f
diff --git a/libc/upstream-freebsd/README.txt b/libc/upstream-freebsd/README.md
similarity index 100%
rename from libc/upstream-freebsd/README.txt
rename to libc/upstream-freebsd/README.md
diff --git a/libc/upstream-netbsd/README.txt b/libc/upstream-netbsd/README.md
similarity index 100%
rename from libc/upstream-netbsd/README.txt
rename to libc/upstream-netbsd/README.md
diff --git a/libc/upstream-openbsd/README.txt b/libc/upstream-openbsd/README.md
similarity index 100%
rename from libc/upstream-openbsd/README.txt
rename to libc/upstream-openbsd/README.md