Provide a README for Claude

Add the beginnings of a readme to help Claude do the right thing.

Signed-off-by: Simon Glass <sjg@chromium.org>

.claude/.gitignore

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+settings.local.json

.claude/README.md

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+# U-Boot Build Instructions for Claude Code
+
+This file contains information about building U-Boot for use with Claude Code.
+
+## Building U-Boot
+
+### Using crosfw (Recommended)
+
+To build U-Boot for sandbox testing, use the `crosfw` command:
+
+```bash
+# Build for sandbox
+crosfw sandbox -L
+
+# The -L flag disables LTO (equivalent to NO_LTO=1)
+# The build is silent unless there are warnings or errors
+# The build is done in /tmp/b/<board_name>, so /tmp/b/sandbox in this case
+```
+
+### Using make directly
+
+If you prefer to use make directly, please use O= to avoid adding build files to
+the source tree:
+
+```bash
+# Clean the build (recommended before first build)
+make mrproper
+
+# Configure for sandbox
+make sandbox_defconfig O=/tmp/<build_dir>
+
+# Build
+make -j$(nproc) O=/tmp/<build_dir>
+```
+## Testing
+
+There are aliases in ~/bin/git-alias which you can use.
+
+To run a sandbox test:
+
+```bash
+rtv <test_name>
+# For example: rtv dm_test_video_box
+# which translates to: /tmp/b/sandbox/u-boot -v -Tf -c "ut dm video_box"
+# test output is silenced unless -v is given
+```
+
+To run using the Python suite:
+
+```bash
+pyt <test_name>
+# alias for: test/py/test.py -B sandbox --build-dir /tmp/b/sandbox -k <test_name>
+```
+
+## Notes
+
+- The `crosfw` tool is the preferred build method for this codebase
+- Always run `make mrproper` if you encounter build issues
+- The sandbox build creates a test environment for U-Boot that runs on the host system
+- When using `git diff`, add `--no-ext-diff` to avoid external diff tools that may not work in this environment
+
+## Coding Conventions
+
+- For function parameters that return values (output parameters), add 'p' suffix to indicate pointer
+  - Example: `uint *sizep` instead of `uint *size`
+  - Example: `bool *visiblep` instead of `bool *visible`
+- This follows U-Boot's established naming convention for output parameters
+- Keep commit messages concise - focus on the key change and essential details only
+- Code should be formatted to 80 columns and not have trailing spaces

.gitignore

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 # Normal rules (sorted alphabetically)
 #
 .*
+!.claude
 !.checkpatch.conf
 *.a
 *.asn1.[ch]